kbuild: fail kernel compilation in case of unresolved module symbols

At stage 2 modpost utility is used to check modules.  In case of unresolved
symbols modpost only prints warning.

IMHO it is a good idea to fail compilation process in case of unresolved
symbols (at least in modules coming with kernel), since usually such errors
are left unnoticed, but kernel modules are broken.

- new option '-w' is added to modpost:
  if option is specified, modpost only warns about unresolved symbols

- modpost is called with '-w' for external modules in Makefile.modpost

Signed-off-by: Andrey Mirkin <amirkin@sw.ru>
Signed-off-by: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/Makefile.modpost

@@ -58,6 +58,7 @@ quiet_cmd_modpost = MODPOST $(words $(filter-out vmlinux FORCE, $^)) modules
 	$(if $(KBUILD_EXTMOD),-i,-o) $(kernelsymfile) \
 	$(if $(KBUILD_EXTMOD),-I $(modulesymfile)) \
 	$(if $(KBUILD_EXTMOD),-o $(modulesymfile)) \
+	$(if $(KBUILD_EXTMOD),-w) \
 	$(wildcard vmlinux) $(filter-out FORCE,$^)
 
 PHONY += __modpost