kbuild: make wrapper Makefile more convenient for external modules

When Kbuild starts building in a separate output directory, it generates
a wrapper Makefile, allowing you to invoke 'make' from the output
directory.

This commit makes it more convenient, so you can invoke 'make' without
M= or MO=.

First, you need to build external modules in a separate directory:

$ make M=/path/to/module/source/dir MO=/path/to/module/build/dir

Once the wrapper Makefile is generated in /path/to/module/build/dir,
you can proceed as follows:

$ cd /path/to/module/build/dir
$ make

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>