[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.

 # Compute x86_64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }