kbuild: Generate modules.builtin in make modules

Generating the file in make modules_install was broken as well, because
it didn't work in a readonly filesystem and otherwise it generated a
root-owned file which is not wanted.
Reported-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -1095,7 +1095,7 @@ all: modules
 #	using awk while concatenating to the final file.
 
 PHONY += modules
-modules: $(vmlinux-dirs) $(if $(KBUILD_BUILTIN),vmlinux)
+modules: $(vmlinux-dirs) $(if $(KBUILD_BUILTIN),vmlinux) modules.builtin
 	$(Q)$(AWK) '!x[$$0]++' $(vmlinux-dirs:%=$(objtree)/%/modules.order) > $(objtree)/modules.order
 	@$(kecho) '  Building modules, stage 2.';
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
@@ -1117,7 +1117,7 @@ PHONY += modules_install
 modules_install: _modinst_ _modinst_post
 
 PHONY += _modinst_
-_modinst_: modules.builtin
+_modinst_:
 	@if [ -z "`$(DEPMOD) -V 2>/dev/null | grep module-init-tools`" ]; then \
 		echo "Warning: you may need to install module-init-tools"; \
 		echo "See http://www.codemonkey.org.uk/docs/post-halloween-2.6.txt";\