[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="unknown"

 reporter_url="http://iphonelinux.org/index.php/Glibc_toolchain_using_crosstool-NG"

 reporter_comment="An (experimental) toolchain targeting the Apple's iPhone."