kbuild: Do not unnecessarily regenerate modules.builtin

Only regenerate it if the configuration has changed. Also, do this after
the modules build to fix errors with some weird Makefiles that are
generated during build.
Reported-by: NEric Miao <eric.y.miao@gmail.com>
Signed-off-by: NMichal Marek <mmarek@suse.cz>

Makefile

@@ -882,9 +882,6 @@ $(sort $(vmlinux-init) $(vmlinux-main)) $(vmlinux-lds): $(vmlinux-dirs) ;
 PHONY += $(vmlinux-dirs)
 $(vmlinux-dirs): prepare scripts
 	$(Q)$(MAKE) $(build)=$@
-ifdef CONFIG_MODULES
-	$(Q)$(MAKE) $(modbuiltin)=$@
-endif
 
 # Build the kernel release string
 #
@@ -1089,7 +1086,7 @@ ifdef CONFIG_MODULES
 
 # By default, build modules as well
 
-all: modules
+all: modules modules.builtin
 
 #	Build modules
 #
@@ -1100,11 +1097,16 @@ all: modules
 PHONY += modules
 modules: $(vmlinux-dirs) $(if $(KBUILD_BUILTIN),vmlinux)
 	$(Q)$(AWK) '!x[$$0]++' $(vmlinux-dirs:%=$(objtree)/%/modules.order) > $(objtree)/modules.order
-	$(Q)$(AWK) '!x[$$0]++' $(vmlinux-dirs:%=$(objtree)/%/modules.builtin) > $(objtree)/modules.builtin
 	@$(kecho) '  Building modules, stage 2.';
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.fwinst obj=firmware __fw_modbuild
 
+modules.builtin: $(vmlinux-dirs:%=%/modules.builtin)
+	$(Q)$(AWK) '!x[$$0]++' $^ > $(objtree)/modules.builtin
+
+%/modules.builtin: include/config/auto.conf | modules
+	$(Q)$(MAKE) $(modbuiltin)=$*
+
 
 # Target to prepare building external modules
 PHONY += modules_prepare