kbuild: support modules_sign for external modules as well

The modules_sign target is currently only available for in-tree modules,
but it actually works for external modules as well.

Move the modules_sign rule to the common part.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Makefile

@@ -1461,20 +1461,6 @@ modules: modules_prepare
 modules_prepare: prepare
 	$(Q)$(MAKE) $(build)=scripts scripts/module.lds
 
-export modules_sign_only :=
-
-ifeq ($(CONFIG_MODULE_SIG),y)
-PHONY += modules_sign
-modules_sign: modules_install
-	@:
-
-# modules_sign is a subset of modules_install.
-# 'make modules_install modules_sign' is equivalent to 'make modules_install'.
-ifeq ($(filter modules_install,$(MAKECMDGOALS)),)
-modules_sign_only := y
-endif
-endif
-
 endif # CONFIG_MODULES
 
 ###
@@ -1833,10 +1819,24 @@ endif # KBUILD_EXTMOD
 # ---------------------------------------------------------------------------
 # Modules
 
-PHONY += modules modules_install modules_prepare
+PHONY += modules modules_install modules_sign modules_prepare
 
 modules_install:
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst \
+	sign-only=$(if $(filter modules_install,$(MAKECMDGOALS)),,y)
+
+ifeq ($(CONFIG_MODULE_SIG),y)
+# modules_sign is a subset of modules_install.
+# 'make modules_install modules_sign' is equivalent to 'make modules_install'.
+modules_sign: modules_install
+	@:
+else
+modules_sign:
+	@echo >&2 '***'
+	@echo >&2 '*** CONFIG_MODULE_SIG is disabled. You cannot sign modules.'
+	@echo >&2 '***'
+	@false
+endif
 
 ifdef CONFIG_MODULES
 

scripts/Makefile.modinst

@@ -11,7 +11,7 @@ include $(srctree)/scripts/Kbuild.include
 
 install-y :=
 
-ifeq ($(KBUILD_EXTMOD)$(modules_sign_only),)
+ifeq ($(KBUILD_EXTMOD)$(sign-only),)
 
 # remove the old directory and symlink
 $(shell rm -fr $(MODLIB)/kernel $(MODLIB)/build)
@@ -104,7 +104,7 @@ quiet_cmd_sign = SIGN    $@
       cmd_sign = scripts/sign-file $(CONFIG_MODULE_SIG_HASH) "$(sig-key)" certs/signing_key.x509 $@ \
                  $(if $(KBUILD_EXTMOD),|| true)
 
-ifeq ($(modules_sign_only),)
+ifeq ($(sign-only),)
 
 # During modules_install, modules are signed only when CONFIG_MODULE_SIG_ALL=y.
 ifndef CONFIG_MODULE_SIG_ALL