From: Sam Ravnborg <sam@ravnborg.org>

When building external modules the $PWD/.tmp_versions directory is used.
The .tmp_versions directory in the kernel tree cannot be used because this
would clutter up the kernel tree especially when more than one external
module is being build for the same kernel tree.

This patch make sure to create $PWD/.tmp_versions, and to delete it during
make clean.  It also removes warning about 'messed with SUBDIRS', this is
no longer relevant when .tmp_versions is made outside the kernel tree.