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

Since commit ff9b45c55b26 ("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: ff9b45c55b26 ("kbuild: modpost: read modules.order instead of $(MODVERDIR)/*.mod")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index a7124f8..915775e 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -210,7 +210,7 @@
 modpost_link vmlinux.o
 
 # modpost vmlinux.o to check for section mismatches
-${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o
+${MAKE} -f "${srctree}/scripts/Makefile.modpost" MODPOST_VMLINUX=1
 
 info MODINFO modules.builtin.modinfo
 ${OBJCOPY} -j .modinfo -O binary vmlinux.o modules.builtin.modinfo