fix linux-headers checkout

The logic in this ebuild fails as it tries to use a local git repo
that is at a different git sha1.  Tweak it so that it'll fall back
to the GoB repo.

BUG=None
TEST=can setup_board for stumpy/lumpy again

Change-Id: I7e8357d55e0af8cf4206b82f1d4a403c250ba834
Reviewed-on: https://chromium-review.googlesource.com/194811
Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed