[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.

# x86_64 specific options

config ARCH_x86_64

    select ARCH_64

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    help

      The x86_64 architecture, as defined by:

        http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html