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.

# dmalloc debug facility

config DEBUG_dmalloc

    bool

choice

    bool

    prompt "dmalloc version"

# Don't remove next line

# CT_INSERT_VERSION_BELOW

config DMALLOC_V_5_5_2

    bool

    prompt "5.5.2"

endchoice

config DMALLOC_VERSION

    string

# Don't remove next line

# CT_INSERT_VERSION_STRING_BELOW

    default "5.5.2" if DMALLOC_V_5_5_2