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.

 source config/global.in

 source config/target.in

 source config/toolchain.in

 source config/kernel.in

 source config/binutils.in

 source config/cc.in

 source config/libc.in

 source config/debug.in

 source config/tools.in

 source config/companion_libs.in

 source config/companion_tools.in