kbuild: modsign: read modules.order instead of $(MODVERDIR)/*.mod

Towards the goal of removing MODVERDIR, read out modules.order to get
the list of modules to be signed. This is simpler than parsing *.mod
files in $(MODVERDIR).

The modules_sign target is only supported for in-kernel modules.
So, this commit does not take care of external modules.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>