kbuild: remove 'make /' support

'make /' is just an alias for 'make ./'; this builds all objects of an
external module, but skips the modpost stage.

I am not a big fan of 'make /' since it looks as if it were touching
the root directory of the system. I like 'make ./' better.

I do not know how many people are using it, but let's show a hint if
it is used. Also, move it close to the external module rules since
this only makes sense for external modules.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1648,6 +1648,10 @@ $(clean-dirs):
 
 clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers
 
+PHONY += /
+/:
+	@echo >&2 '"$(MAKE) /" is no longer supported. Please use "$(MAKE) ./" instead.'
+
 PHONY += help
 help:
 	@echo  '  Building external modules.'
@@ -1783,11 +1787,6 @@ endif
 	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target:.ko=.mod)
 	$(Q)echo $(build-target) > $(MODORDER)
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
-
-# Modules
-PHONY += /
-/: ./
-
 %/: prepare FORCE
 	$(Q)$(MAKE) KBUILD_MODULES=1 $(build)=$(build-dir) need-modorder=1