Makefile: install modules.builtin even if CONFIG_MODULES=n

Many applications check for available kernel features via:

  - /proc/modules (loaded modules, present if CONFIG_MODULES=y)
  - $(MODLIB)/modules.builtin (builtin modules)

They fail to detect features if the kernel was built with CONFIG_MODULES=n
and modules.builtin isn't installed.

Therefore, add the target "_builtin_inst_" and make "install" and
"modules_install" depend on it.

Tests results:

  - make install: kernel image is copied as before, modules.builtin copied
  - make modules_install: (CONFIG_MODULES=n) nothing is copied, exit 1
Signed-off-by: Jonas Zeiger <jonas.zeiger@talpidae.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1322,6 +1322,16 @@ dt_binding_check: scripts_dtc
 # ---------------------------------------------------------------------------
 # Modules
 
+# install modules.builtin regardless of CONFIG_MODULES
+PHONY += _builtin_inst_
+_builtin_inst_:
+	@mkdir -p $(MODLIB)/
+	@cp -f modules.builtin $(MODLIB)/
+	@cp -f $(objtree)/modules.builtin.modinfo $(MODLIB)/
+
+PHONY += install
+install: _builtin_inst_
+
 ifdef CONFIG_MODULES
 
 # By default, build modules as well
@@ -1365,7 +1375,7 @@ PHONY += modules_install
 modules_install: _modinst_ _modinst_post
 
 PHONY += _modinst_
-_modinst_:
+_modinst_: _builtin_inst_
 	@rm -rf $(MODLIB)/kernel
 	@rm -f $(MODLIB)/source
 	@mkdir -p $(MODLIB)/kernel
@@ -1375,8 +1385,6 @@ _modinst_:
 		ln -s $(CURDIR) $(MODLIB)/build ; \
 	fi
 	@sed 's:^:kernel/:' modules.order > $(MODLIB)/modules.order
-	@cp -f modules.builtin $(MODLIB)/
-	@cp -f $(objtree)/modules.builtin.modinfo $(MODLIB)/
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst
 
 # This depmod is only for convenience to give the initial