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/