cros_generate_update_payload: Use loopbacks instead of copying partitions.

Instead of copying around partitions, use loopback devices with partition
scanning to refer to specific partitions.

CQ-DEPEND=CL:223113
BUG=None
TEST=Ran a butterfly-paladin tryjob. Ran paygen_payload with old and new
payloads.

Change-Id: Ife003c090ce655d814b1e9c49707545ee938e841
Reviewed-on: https://chromium-review.googlesource.com/223128
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
1 file changed