update_payload: Add the remaining major version 2 signature supports

This patch fixes the issues with signatures (sizes) in major version 2 and a few
minor issues with the payload and metadata sizes.

BUG=chromium:862679
TEST=manually signing the payload and running update_payload_check
TEST=unittests

Change-Id: I9b431379b0574a150474a913f1ec4a11e86288ae
Reviewed-on: https://chromium-review.googlesource.com/1363339


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