kbuild: modpost: do not parse unnecessary rules for vmlinux modpost

Since commit ff9b45c5 ("kbuild: modpost: read modules.order instead
of $(MODVERDIR)/*.mod"), 'make vmlinux' emits a warning, like this:

$ make defconfig vmlinux
  [ snip ]
  LD      vmlinux.o
cat: modules.order: No such file or directory
  MODPOST vmlinux.o
  MODINFO modules.builtin.modinfo
  KSYM    .tmp_kallsyms1.o
  KSYM    .tmp_kallsyms2.o
  LD      vmlinux
  SORTEX  vmlinux
  SYSMAP  System.map

When building only vmlinux, KBUILD_MODULES is not set. Hence, the
modules.order is not generated. For the vmlinux modpost, it is not
necessary at all.

Separate scripts/Makefile.modpost for the vmlinux/modules stages.
This works more efficiently because the vmlinux modpost does not
need to include .*.cmd files.

Fixes: ff9b45c5 ("kbuild: modpost: read modules.order instead of $(MODVERDIR)/*.mod")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>