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.

 # 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