[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.

# ia64 specific config options

# depends on EXPERIMENTAL

config ARCH_ia64

    select ARCH_USE_MMU

    select ARCH_64

    select ARCH_SUPPORTS_BOTH_ENDIAN

    help

      The ia64 architecture, as defined by:

        http://www.intel.com/design/itanium/arch_spec.htm