Add tool to diff two target files packages

This tool will help identify unexpected differences between two target
files packages. Expected changes (build number / date) are hidden, along
with known bugs.

It ignores anything that is just packaging for other files that are
already being diffed, like the recovery patching, and all of IMAGES/*

Bug 25372309

Change-Id: I4641754dd5dfbdfd3a89d8fa5262b9a9595e8e9c