update_payload: Add puffdiff support.

This patch adds support for applier.py to be able to apply PUFFDIFF
operation using puffin binary. It also fixes minor deficiencies for the
PUFFDIFF operation.

BUG=chromium:768461
TEST=unittests pass;
TEST=scripts/paycheck.py payload-puffin.delta new_kern.dat new_root.dat old_kern.dat old_root.dat

Change-Id: I5612ead8d8b8176e7263cfcb15403e8d36319642
Reviewed-on: https://chromium-review.googlesource.com/713540


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>