config: prepare for build-system backend

When acting as a backend for a build-system, we should not build
any application that runs on the target, that is:
- no native gdb
- no companion libraries
- no binutils libraries
- no debug tools (save for gdbserver)
- ...

Here, we simply prepare the (hidden) config option that will detect
that we are acting as a back-end.

Update doc accordingly.

config/global/ct-behave.in

@@ -2,6 +2,15 @@
 
 comment "crosstool-NG behavior"
 
+config IS_A_BACKEND
+    string
+    option env="CT_IS_A_BACKEND"
+
+config BACKEND
+    bool
+    default y if IS_A_BACKEND =  "y" || IS_A_BACKEND =  "Y"
+    default n if IS_A_BACKEND != "y" && IS_A_BACKEND != "Y"
+
 config OBSOLETE
     bool
     prompt "Use obsolete features"

docs/overview.txt

@@ -21,6 +21,7 @@ Installing crosstool-NG
 Configuring crosstool-NG
   Interesting config options
   Re-building an existing toolchain
+  Using as a backend for a build-system
 Running crosstool-NG
   Stopping and restarting a build
   Testing all toolchains at once
@@ -350,6 +351,21 @@ toolchain with this configuration, just redirect the output to the
 Then, you can review and change the configuration by running:
   ct-ng menuconfig
 
+Using as a backend for a build-system |
+--------------------------------------+
+
+Crosstool-NG can be used as a backend for an automated build-system. In this
+case, some components that are expected to run on the target (eg. the native
+gdb, ltrace, DUMA...) are not available in the menuconfig, and they are not
+build either, as it is considered the responsibility of the build-system to
+build its own versions of those tools.
+
+If you want to use crosstool-NG as a backend to generate your toolchains for
+your build-system, you have to set and export this environment variable:
+  CT_IS_A_BACKEND=y
+
+(case is not sensitive, you can say Y).
+
 
 ________________________
                        /