[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.

# Compute x86_64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

}