[config] Warn againt using ash as CONFIG_SHELL

It apears that more and more ./configure scripts and Makefiles
make use of non-POSIXly correct shell constructs, that don't
work with ash.

For now, just warn the user against using ash, but keep it as
an option in case newer versions of the /broken/ components
come fixed, and we can again use ash, as it is in some cases
really faster than bash.

 source config/global.in

 source config/target.in

 source config/toolchain.in

 source config/kernel.in

 source config/companion_libs.in

 source config/binutils.in

 source config/cc.in

 source config/libc.in

 source config/tools.in

 source config/debug.in