kbuild: make modules_install copy modules.builtin(.modinfo)

Josh Triplett reports that initramfs-tools needs modules.builtin and
modules.builtin.modinfo to create a working initramfs for a non-modular
kernel.

If this is a general tooling issue not limited to Debian, I think it
makes sense to change modules_install.

This commit changes the targets as follows when CONFIG_MODULES=n.

In-tree builds:
  make modules          -> no-op
  make modules_install  -> install modules.builtin(.modinfo)

External module builds:
  make modules          -> show error message like before
  make modules_install  -> show error message like before

Link: https://lore.kernel.org/lkml/36a4014c73a52af27d930d3ca31d362b60f4461c.1686356364.git.josh@joshtriplett.org/
Reported-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Tested-by: Josh Triplett <josh@joshtriplett.org>

Makefile

@@ -1545,6 +1545,8 @@ modules_sign_only := y
 endif
 endif
 
+endif # CONFIG_MODULES
+
 modinst_pre :=
 ifneq ($(filter modules_install,$(MAKECMDGOALS)),)
 modinst_pre := __modinst_pre
@@ -1555,18 +1557,18 @@ PHONY += __modinst_pre
 __modinst_pre:
 	@rm -rf $(MODLIB)/kernel
 	@rm -f $(MODLIB)/source
-	@mkdir -p $(MODLIB)/kernel
+	@mkdir -p $(MODLIB)
+ifdef CONFIG_MODULES
 	@ln -s $(abspath $(srctree)) $(MODLIB)/source
 	@if [ ! $(objtree) -ef  $(MODLIB)/build ]; then \
 		rm -f $(MODLIB)/build ; \
 		ln -s $(CURDIR) $(MODLIB)/build ; \
 	fi
 	@sed 's:^\(.*\)\.o$$:kernel/\1.ko:' modules.order > $(MODLIB)/modules.order
+endif
 	@cp -f modules.builtin $(MODLIB)/
 	@cp -f $(objtree)/modules.builtin.modinfo $(MODLIB)/
 
-endif # CONFIG_MODULES
-
 ###
 # Cleaning is done on three levels.
 # make clean     Delete most generated files
@@ -1908,6 +1910,13 @@ help:
 	@echo  '  clean           - remove generated files in module directory only'
 	@echo  ''
 
+__external_modules_error:
+	@echo >&2 '***'
+	@echo >&2 '*** The present kernel disabled CONFIG_MODULES.'
+	@echo >&2 '*** You cannot build or install external modules.'
+	@echo >&2 '***'
+	@false
+
 endif # KBUILD_EXTMOD
 
 # ---------------------------------------------------------------------------
@@ -1944,13 +1953,10 @@ else # CONFIG_MODULES
 # Modules not configured
 # ---------------------------------------------------------------------------
 
-modules modules_install:
-	@echo >&2 '***'
-	@echo >&2 '*** The present kernel configuration has modules disabled.'
-	@echo >&2 '*** To use the module feature, please run "make menuconfig" etc.'
-	@echo >&2 '*** to enable CONFIG_MODULES.'
-	@echo >&2 '***'
-	@exit 1
+PHONY += __external_modules_error
+
+modules modules_install: __external_modules_error
+	@:
 
 KBUILD_MODULES :=