kbuild: merge scripts/Makefile.modsign to scripts/Makefile.modinst

scripts/Makefile.modsign is a subset of scripts/Makefile.modinst,
and duplicates the code. Let's merge them.

By the way, you do not need to run 'make modules_sign' explicitly
because modules are signed as a part of 'make modules_install' when
CONFIG_MODULE_SIG_ALL=y. If CONFIG_MODULE_SIG_ALL=n, mod_sign_cmd is
set to 'true', so 'make modules_sign' is not functional.

In my understanding, the reason of still keeping this is to handle
corner cases like commit 64178cb6

 ("builddeb: fix stripped module
signatures if CONFIG_DEBUG_INFO and CONFIG_MODULE_SIG_ALL are set").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>