[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="Julien DUSSER"

reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00054.html"

reporter_comment="Soft-float for the 405 PowerPC"