update_payload: Allow paycheck.py to compare apply results.

Currently paycheck.py only applies the payloads. The responsibility of checking
if the final root and kernel partitions are what we wanted is out of
paycheck.py. Currently when paycheck.py runs and applies a payload files passed
by --dst_kern and --dst_root are populated with the newly generated partitions
by applier.py. This patch adds two new flags --out_dst_kern and --out_dst_root
so instead paychek.py outputs the new partitions into paths passed by these
flags and --dst_kern and --dst_root is reserved for passing the actual original
partitions that newly generated partitions need to be compared to. This allows
paycheck.py to check the newly generated partitions against the given ones and
make sure they are equal.

BUG=chromium:808495
TEST=test_paycheck.sh old.full delta.bin new.full

Change-Id: I57a476de1477e3cba7557d0564b49ec03253c047
Reviewed-on: https://chromium-review.googlesource.com/929652


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