kbuild: turn '/' into an alias of './'

Commit 06300b21f4c7 ("kbuild: support building individual files for
external modules") introduced the '/' target. It works only for
external modules to build all .o files, but skip the modpost stage.

However, 'make /' looks a bit weird to me. 'make ./' is more sensible
if you want to build all objects under the current directory, and it
works as expected.

Let's change '/' into a phony target that is an alias of './', but
I may feel like deprecating it in the future.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
index cacb751..8d50275 100644
--- a/Makefile
+++ b/Makefile
@@ -1715,8 +1715,9 @@
 	$(Q)$(MAKE) $(build)=$(build-dir) $(target-dir)$(notdir $@)
 
 # Modules
-/: prepare FORCE
-	$(Q)$(MAKE) KBUILD_MODULES=1 $(build)=$(build-dir)
+PHONY += /
+/: ./
+
 # Make sure the latest headers are built for Documentation
 Documentation/ samples/: headers_install
 %/: prepare FORCE