[config] Change the way to select the CONFIG_SHELL

Transform the single boolean option to a 'choice' selection
with two options: "sh" and "ash".

This is needed to later add "bash" as override.

 reporter_name="Matthias KAEHLCKE"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-08/msg00055.html"

 reporter_comment=""