Combine checksum of all boot images.

Allows a change to be detected in more than just the first image.
Combines checksums of all boot images using xor. A better hash is left
as TODO.

Bug: 28054110

(cherry-picked from commit 4f351aae00f43dd4e3290976f249d3664cc47091)

Change-Id: Ia557e2460ca582082368ef93508c04ce32c3036a
4 files changed