patman: symlink top level .checkpatch.conf

This makes it possible to run the patman test suite simply by invoking
'pytest' from the patman sub-directory:

  $ cd tools/patman
  $ pytest

Otherwise, the top level .checkpatch.conf would be ignored and
multiple test_checkpatch.py tests would fail.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>
Reviewed-by: Simon Glass <sjg@chromium.org>