kbuild: stop using config_filename in scripts/Makefile.modsign

Toward the goal of removing the config_filename macro, drop
the double-quotes and add $(srctree)/ prefix in an ad hoc way.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>