kbuild: move 'PHONY += modules_prepare' to the common part

Unify the code between in-tree builds and external module builds.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1441,7 +1441,6 @@ modules.order: $(subdir-modorder) FORCE
 targets += modules.order
 
 # Target to prepare building external modules
-PHONY += modules_prepare
 modules_prepare: prepare
 	$(Q)$(MAKE) $(build)=scripts scripts/module.lds
 
@@ -1742,15 +1741,12 @@ help:
 	@echo  '  clean           - remove generated files in module directory only'
 	@echo  ''
 
-# no-op for external module builds
-PHONY += modules_prepare
-
 endif # KBUILD_EXTMOD
 
 # ---------------------------------------------------------------------------
 # Modules
 
-PHONY += modules modules_install
+PHONY += modules modules_install modules_prepare
 
 ifdef CONFIG_MODULES