kbuild: re-run modpost when it is updated

Modpost generates .vmlinux.export.c and *.mod.c, which are prerequisites
of vmlinux and modules, respectively.

The modpost stage should be re-run when the modpost code is updated.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.modpost

@@ -38,7 +38,7 @@ __modpost:
 include include/config/auto.conf
 include $(srctree)/scripts/Kbuild.include
 
-MODPOST = scripts/mod/modpost								\
+modpost-args =										\
 	$(if $(CONFIG_MODVERSIONS),-m)							\
 	$(if $(CONFIG_MODULE_SRCVERSION_ALL),-a)					\
 	$(if $(CONFIG_SECTION_MISMATCH_WARN_ONLY),,-E)					\
@@ -48,7 +48,7 @@ MODPOST = scripts/mod/modpost								\
 
 # 'make -i -k' ignores compile errors, and builds as many modules as possible.
 ifneq ($(findstring i,$(filter-out --%,$(MAKEFLAGS))),)
-MODPOST += -n
+modpost-args += -n
 endif
 
 ifeq ($(KBUILD_EXTMOD),)
@@ -100,16 +100,18 @@ module.symvers-if-present := $(wildcard Module.symvers)
 output-symdump := $(KBUILD_EXTMOD)/Module.symvers
 missing-input := $(filter-out $(module.symvers-if-present), Module.symvers)
 
-MODPOST += -e $(addprefix -i ,$(module.symvers-if-present) $(KBUILD_EXTRA_SYMBOLS))
+modpost-args += -e $(addprefix -i ,$(module.symvers-if-present) $(KBUILD_EXTRA_SYMBOLS))
 
 endif # ($(KBUILD_EXTMOD),)
 
 ifneq ($(KBUILD_MODPOST_WARN)$(missing-input),)
-MODPOST += -w
+modpost-args += -w
 endif
 
 modorder-if-needed := $(if $(KBUILD_MODULES), $(MODORDER))
 
+MODPOST = scripts/mod/modpost
+
 # Read out modules.order to pass in modpost.
 # Otherwise, allmodconfig would fail with "Argument list too long".
 quiet_cmd_modpost = MODPOST $@
@@ -118,10 +120,10 @@ quiet_cmd_modpost = MODPOST $@
 		echo >&2 "WARNING: $(missing-input) is missing."; \
 		echo >&2 "         Modules may not have dependencies or modversions."; \
 		echo >&2 "         You may get many unresolved symbol warnings.";) \
-	sed 's/ko$$/o/' $(or $(modorder-if-needed), /dev/null) | $(MODPOST) $(vmlinux.o-if-present) -T -
+	sed 's/ko$$/o/' $(or $(modorder-if-needed), /dev/null) | $(MODPOST) $(modpost-args) $(vmlinux.o-if-present) -T -
 
 targets += $(output-symdump)
-$(output-symdump): $(modorder-if-needed) $(vmlinux.o-if-present) $(moudle.symvers-if-present) FORCE
+$(output-symdump): $(modorder-if-needed) $(vmlinux.o-if-present) $(moudle.symvers-if-present) $(MODPOST) FORCE
 	$(call if_changed,modpost)
 
 __modpost: $(output-symdump)