modpost: show warning if any of symbol dump files is missing

If modpost fails to load a symbol dump file, it cannot check unresolved
symbols, hence module dependency will not be added. Nor CRCs can be added.

Currently, external module builds check only $(objtree)/Module.symvers,
but it should check files specified by KBUILD_EXTRA_SYMBOLS as well.

Move the warning message from the top Makefile to scripts/Makefile.modpost
and print the warning if any dump file is missing.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1649,17 +1649,9 @@ else # KBUILD_EXTMOD
 # We are always building modules
 KBUILD_MODULES := 1
 
-PHONY += $(objtree)/Module.symvers
-$(objtree)/Module.symvers:
-	@test -e $(objtree)/Module.symvers || ( \
-	echo; \
-	echo "  WARNING: Symbol version dump $(objtree)/Module.symvers"; \
-	echo "           is missing; modules will have no dependencies and modversions."; \
-	echo )
-
 build-dirs := $(KBUILD_EXTMOD)
 PHONY += modules
-modules: descend $(objtree)/Module.symvers
+modules: descend
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 
 PHONY += modules_install

scripts/Makefile.modpost

@@ -98,8 +98,11 @@ ifneq ($(findstring i,$(filter-out --%,$(MAKEFLAGS))),)
 MODPOST += -n
 endif
 
+# Clear VPATH to not search for *.symvers in $(srctree). Check only $(objtree).
+VPATH :=
 $(input-symdump):
-	@:
+	@echo >&2 'WARNING: Symbol version dump "$@" is missing.'
+	@echo >&2 '         Modules may not have dependencies or modversions.'
 
 # Read out modules.order to pass in modpost.
 # Otherwise, allmodconfig would fail with "Argument list too long".