restore documentation on the build config language

Kconfig-language.txt was deleted in commit 4fa499a17b52b back in 2006.
Move to docs/ as suggested by Xabier Oneca:
  http://lists.busybox.net/pipermail/busybox/2014-May/080914.html
Also update references to it everywhere.

Signed-off-by: Kartik Agaram <akkartik@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

25 files changed, 279 insertions, 24 deletions

diff --git a/Config.in b/Config.in
index 51ff01ef4..ae21f52ef 100644
--- a/Config.in
+++ b/Config.in
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 mainmenu "Configuration"
diff --git a/archival/Config.src b/archival/Config.src
index 449914565..6f4f30c43 100644
--- a/archival/Config.src
+++ b/archival/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Archival Utilities"
diff --git a/console-tools/Config.src b/console-tools/Config.src
index e6587ade4..c30caf0e1 100644
--- a/console-tools/Config.src
+++ b/console-tools/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Console Utilities"
diff --git a/coreutils/Config.src b/coreutils/Config.src
index 7a8a3a634..1bded03a6 100644
--- a/coreutils/Config.src
+++ b/coreutils/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Coreutils"
diff --git a/debianutils/Config.src b/debianutils/Config.src
index 61daeb047..17b0d8945 100644
--- a/debianutils/Config.src
+++ b/debianutils/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Debian Utilities"
diff --git a/docs/Kconfig-language.txt b/docs/Kconfig-language.txt
new file mode 100644
index 000000000..0ba8932ba
--- /dev/null
+++ b/docs/Kconfig-language.txt
@@ -0,0 +1,255 @@
+Introduction
+------------
+
+The configuration database is collection of configuration options
+organized in a tree structure:
+
+    +- Code maturity level options
+    |  +- Prompt for development and/or incomplete code/drivers
+    +- General setup
+    |  +- Networking support
+    |  +- System V IPC
+    |  +- BSD Process Accounting
+    |  +- Sysctl support
+    +- Loadable module support
+    |  +- Enable loadable module support
+    |     +- Set version information on all module symbols
+    |     +- Kernel module loader
+    +- ...
+
+Every entry has its own dependencies. These dependencies are used
+to determine the visible of an entry. Any child entry is only
+visible if its parent entry is also visible.
+
+Menu entries
+------------
+
+Most entries define a config option, all other entries help to organize
+them. A single configuration option is defined like this:
+
+config MODVERSIONS
+    bool "Set version information on all module symbols"
+    depends MODULES
+    help
+      Usually, modules have to be recompiled whenever you switch to a new
+      kernel.  ...
+
+Every line starts with a key word and can be followed by multiple
+arguments.  "config" starts a new config entry. The following lines
+define attributes for this config option. Attributes can be the type of
+the config option, input prompt, dependencies, help text and default
+values. A config option can be defined multiple times with the same
+name, but every definition can have only a single input prompt and the
+type must not conflict.
+
+Menu attributes
+---------------
+
+A menu entry can have a number of attributes. Not all of them are
+applicable everywhere (see syntax).
+
+- type definition: "bool"/"tristate"/"string"/"hex"/"integer"
+  Every config option must have a type. There are only two basic types:
+  tristate and string, the other types base on these two. The type
+  definition optionally accepts an input prompt, so these two examples
+  are equivalent:
+
+    bool "Networking support"
+  and
+    bool
+    prompt "Networking support"
+
+- input prompt: "prompt" <prompt> ["if" <expr>]
+  Every menu entry can have at most one prompt, which is used to display
+  to the user. Optionally dependencies only for this prompt can be added
+  with "if".
+
+- default value: "default" <symbol> ["if" <expr>]
+  A config option can have any number of default values. If multiple
+  default values are visible, only the first defined one is active.
+  Default values are not limited to the menu entry, where they are
+  defined, this means the default can be defined somewhere else or be
+  overridden by an earlier definition.
+  The default value is only assigned to the config symbol if no other
+  value was set by the user (via the input prompt above). If an input
+  prompt is visible the default value is presented to the user and can
+  be overridden by him.
+  Optionally dependencies only for this default value can be added with
+  "if".
+
+- dependencies: "depends on"/"requires" <expr>
+  This defines a dependency for this menu entry. If multiple
+  dependencies are defined they are connected with '&&'. Dependencies
+  are applied to all other options within this menu entry (which also
+  accept "if" expression), so these two examples are equivalent:
+
+    bool "foo" if BAR
+    default y if BAR
+  and
+    depends on BAR
+    bool "foo"
+    default y
+
+- help text: "help"
+  This defines a help text. The end of the help text is determined by
+  the level indentation, this means it ends at the first line which has
+  a smaller indentation than the first line of the help text.
+
+
+Menu dependencies
+-----------------
+
+Dependencies define the visibility of a menu entry and can also reduce
+the input range of tristate symbols. The tristate logic used in the
+expressions uses one more state than normal boolean logic to express the
+module state. Dependency expressions have the following syntax:
+
+<expr> ::= <symbol>                             (1)
+           <symbol> '=' <symbol>                (2)
+           <symbol> '!=' <symbol>               (3)
+           '(' <expr> ')'                       (4)
+           '!' <expr>                           (5)
+           <expr> '||' <expr>                   (6)
+           <expr> '&&' <expr>                   (7)
+
+Expressions are listed in decreasing order of precedence.
+
+(1) Convert the symbol into an expression. Boolean and tristate symbols
+    are simply converted into the respective expression values. All
+    other symbol types result in 'n'.
+(2) If the values of both symbols are equal, it returns 'y',
+    otherwise 'n'.
+(3) If the values of both symbols are equal, it returns 'n',
+    otherwise 'y'.
+(4) Returns the value of the expression. Used to override precedence.
+(5) Returns the result of (2-/expr/).
+(6) Returns the result of min(/expr/, /expr/).
+(7) Returns the result of max(/expr/, /expr/).
+
+An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
+respectively for calculations). A menu entry becomes visible when it's
+expression evaluates to 'm' or 'y'.
+
+There are two type of symbols: constant and nonconstant symbols.
+Nonconstant symbols are the most common ones and are defined with the
+'config' statement. Nonconstant symbols consist entirely of alphanumeric
+characters or underscores.
+Constant symbols are only part of expressions. Constant symbols are
+always surrounded by single or double quotes. Within the quote any
+other character is allowed and the quotes can be escaped using '\'.
+
+Menu structure
+--------------
+
+The position of a menu entry in the tree is determined in two ways. First
+it can be specified explicitely:
+
+menu "Network device support"
+    depends NET
+
+config NETDEVICES
+    ...
+
+endmenu
+
+All entries within the "menu" ... "endmenu" block become a submenu of
+"Network device support". All subentries inherit the dependencies from
+the menu entry, e.g. this means the dependency "NET" is added to the
+dependency list of the config option NETDEVICES.
+
+The other way to generate the menu structure is done by analyzing the
+dependencies. If a menu entry somehow depends on the previous entry, it
+can be made a submenu of it. First the the previous (parent) symbol must
+be part of the dependency list and then one of these two condititions
+must be true:
+- the child entry must become invisible, if the parent is set to 'n'
+- the child entry must only be visible, if the parent is visible
+
+config MODULES
+    bool "Enable loadable module support"
+
+config MODVERSIONS
+    bool "Set version information on all module symbols"
+    depends MODULES
+
+comment "module support disabled"
+    depends !MODULES
+
+MODVERSIONS directly depends on MODULES, this means it's only visible if
+MODULES is different from 'n'. The comment on the other hand is always
+visible when MODULES it's visible (the (empty) dependency of MODULES is
+also part of the comment dependencies).
+
+
+Kconfig syntax
+--------------
+
+The configuration file describes a series of menu entries, where every
+line starts with a keyword (except help texts). The following keywords
+end a menu entry:
+- config
+- choice/endchoice
+- comment
+- menu/endmenu
+- if/endif
+- source
+The first four also start the definition of a menu entry.
+
+config:
+
+    "config" <symbol>
+    <config options>
+
+This defines a config symbol <symbol> and accepts any of above
+attributes as options.
+
+choices:
+
+    "choice"
+    <choice options>
+    <choice block>
+    "endchoice"
+
+This defines a choice group and accepts any of above attributes as
+options. A choice can only be of type bool or tristate, while a boolean
+choice only allows a single config entry to be selected, a tristate
+choice also allows any number of config entries to be set to 'm'. This
+can be used if multiple drivers for a single hardware exists and only a
+single driver can be compiled/loaded into the kernel, but all drivers
+can be compiled as modules.
+A choice accepts another option "optional", which allows to set the
+choice to 'n' and no entry needs to be selected.
+
+comment:
+
+    "comment" <prompt>
+    <comment options>
+
+This defines a comment which is displayed to the user during the
+configuration process and is also echoed to the output files. The only
+possible options are dependencies.
+
+menu:
+
+    "menu" <prompt>
+    <menu options>
+    <menu block>
+    "endmenu"
+
+This defines a menu block, see "Menu structure" above for more
+information. The only possible options are dependencies.
+
+if:
+
+    "if" <expr>
+    <if block>
+    "endif"
+
+This defines an if block. The dependency expression <expr> is appended
+to all enclosed menu entries.
+
+source:
+
+    "source" <prompt>
+
+This reads the specified configuration file. This file is always parsed.
diff --git a/e2fsprogs/Config.src b/e2fsprogs/Config.src
index a20d849e6..ad15f470c 100644
--- a/e2fsprogs/Config.src
+++ b/e2fsprogs/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Linux Ext2 FS Progs"
diff --git a/editors/Config.src b/editors/Config.src
index 0920bc494..3b2e4a6c0 100644
--- a/editors/Config.src
+++ b/editors/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Editors"
diff --git a/findutils/Config.src b/findutils/Config.src
index 9ee71a845..c28c5844e 100644
--- a/findutils/Config.src
+++ b/findutils/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Finding Utilities"
diff --git a/init/Config.src b/init/Config.src
index 5767c93f0..b19b0bea1 100644
--- a/init/Config.src
+++ b/init/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Init Utilities"
diff --git a/klibc-utils/Config.src b/klibc-utils/Config.src
index fe7cb1315..cf4552e51 100644
--- a/klibc-utils/Config.src
+++ b/klibc-utils/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "klibc-utils"
diff --git a/libbb/Config.src b/libbb/Config.src
index 16e16480b..312aa1831 100644
--- a/libbb/Config.src
+++ b/libbb/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 comment "Library Tuning"
diff --git a/loginutils/Config.src b/loginutils/Config.src
index 680f42118..cbb09646b 100644
--- a/loginutils/Config.src
+++ b/loginutils/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Login/Password Management Utilities"
diff --git a/miscutils/Config.src b/miscutils/Config.src
index 7325fb8fa..d10b00b28 100644
--- a/miscutils/Config.src
+++ b/miscutils/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Miscellaneous Utilities"
diff --git a/modutils/Config.src b/modutils/Config.src
index e413702bb..188296814 100644
--- a/modutils/Config.src
+++ b/modutils/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Linux Module Utilities"
diff --git a/networking/Config.src b/networking/Config.src
index 492c60da4..2ce5287de 100644
--- a/networking/Config.src
+++ b/networking/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Networking Utilities"
diff --git a/networking/udhcp/Config.src b/networking/udhcp/Config.src
index 50bff2e8c..e5958804b 100644
--- a/networking/udhcp/Config.src
+++ b/networking/udhcp/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 config UDHCPD
diff --git a/printutils/Config.src b/printutils/Config.src
index e53b9d093..5f1d65f6c 100644
--- a/printutils/Config.src
+++ b/printutils/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Print Utilities"
diff --git a/procps/Config.src b/procps/Config.src
index 515d79938..2b1b8ab11 100644
--- a/procps/Config.src
+++ b/procps/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Process Utilities"
diff --git a/runit/Config.src b/runit/Config.src
index 8cde89680..403ec8724 100644
--- a/runit/Config.src
+++ b/runit/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Runit Utilities"
diff --git a/selinux/Config.src b/selinux/Config.src
index 9cb755a0f..f8fcdadf9 100644
--- a/selinux/Config.src
+++ b/selinux/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "SELinux Utilities"
diff --git a/shell/Config.src b/shell/Config.src
index 81c4ec874..959d3cb42 100644
--- a/shell/Config.src
+++ b/shell/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Shells"
diff --git a/sysklogd/Config.src b/sysklogd/Config.src
index 684e7d414..321be0117 100644
--- a/sysklogd/Config.src
+++ b/sysklogd/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "System Logging Utilities"
diff --git a/util-linux/Config.src b/util-linux/Config.src
index 68fcc266f..0fad3e5c0 100644
--- a/util-linux/Config.src
+++ b/util-linux/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "Linux System Utilities"
diff --git a/util-linux/volume_id/Config.src b/util-linux/volume_id/Config.src
index ac208c9cc..fe3b14a71 100644
--- a/util-linux/volume_id/Config.src
+++ b/util-linux/volume_id/Config.src
@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 config VOLUMEID