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>