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>

# Default test suite configuration

# Default DejaGnu configuration

DG_TOOLNAME = gcc

DG_TARGET_HOSTNAME = 127.0.0.1

DG_TARGET_USERNAME = root

# Default tests

DG_C_TESTS = 

DG_CPP_TESTS =