kbuild: support building external modules in a separate build directory

There has been a long-standing request to support building external
modules in a separate build directory.

This commit introduces a new environment variable, KBUILD_EXTMOD_OUTPUT,
and its shorthand Make variable, MO.

A simple usage:

 $ make -C <kernel-dir> M=<module-src-dir> MO=<module-build-dir>

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