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: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

diff --git a/Makefile b/Makefile
index 2e74a68..160cada 100644
--- a/Makefile
+++ b/Makefile
@@ -882,9 +882,6 @@
 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 @@
 
 # By default, build modules as well
 
-all: modules
+all: modules modules.builtin
 
 #	Build modules
 #
@@ -1100,11 +1097,16 @@
 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