config: move multi-line shell commands from Makefile to script

Maintaining thos multi-line shell commands in a Makefile rule is
a real PITA.

Move the two affected rules (build_gen_choice_in and build_gen_menu_in
to a shell script.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# m68k specific configuration file

## depends on EXPERIMENTAL

##

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_DEFAULT_BE

## select ARCH_SUPPORT_CPU

##

## help The m68k architecture