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

Commit 06300b21 ("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>

Documentation/kbuild/modules.txt

@@ -140,7 +140,7 @@ executed to make module versioning work.
 		make -C $KDIR M=$PWD bar.lst
 		make -C $KDIR M=$PWD baz.o
 		make -C $KDIR M=$PWD foo.ko
-		make -C $KDIR M=$PWD /
+		make -C $KDIR M=$PWD ./
 
 
 === 3. Creating a Kbuild File for an External Module

Makefile

@@ -1715,8 +1715,9 @@ endif
 	$(Q)$(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