config: make CONFIG_SHELL default to bash

Some components (eg. GMP) will fail to correctly build if
the CONFIG_SHELL is not bash (eg. ash or dash). So make bash
the default CONFIG_SHELL.

Keep ash as a possible selection, as future versions of those
currently /broken/ tools may come fixed wrt to CONFIG_SHELL
being POSIX-ly compliant.

# MIPS specific config options

config ARCH_mips

    select ARCH_USE_MMU

    select ARCH_SUPPORTS_BOTH_ENDIAN

    select ARCH_DEFAULT_BE

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_ABI

    select ARCH_SUPPORT_TUNE

    help

      The MIPS architecture, as defined by:

        http://www.mips.com/