kbuild: Add config fragment merge functionality

So far this function was only used locally in powerpc, some other
architectures might benefit from it. Move it into
scripts/Makefile.defconf.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230124110213.3221264-10-alexander.stein@ew.tq-group.com


Signed-off-by: Arnd Bergmann <arnd@arndb.de>