Paygen: Accept (but ignore) --patch_kernel.

To allow command line compatibility with old versions of this script, we
need to maintain this command line option, even if we ignore it.

BUG=chromium:414322
TEST=None

Change-Id: Id254a1fc3fed6d0a3adc752b716b0e42461ab8c7
Reviewed-on: https://chromium-review.googlesource.com/218444
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>