image_signing: sign_official_build.sh: use loopbacks when updating recovery kernels

This avoids copying in/out the kernels for their configs and to resign.

BRANCH=None
BUG=chromium:714598
TEST=signing images still works

Change-Id: Id13d5099da7f8a73ebd4d4e918188c7eb5b65a12
Reviewed-on: https://chromium-review.googlesource.com/505478
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>
1 file changed