kbuild: modinst: 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 installed. This is simpler than parsing *.mod
files in $(MODVERDIR).

For external modules, $(KBUILD_EXTMOD)/modules.order should be read.

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