Adds new merge builds script for use in merging two non-dist builds.

Bug: 137853921
Test: python -m unittest test_common
Test: python -m unittest test_merge_target_files
Test: Built two partial builds without dist. Ran out/host/linux-x86/bin/merge_builds.
Flashed using `fastboot flashall`. Device boots.
Change-Id: Iffd0a447cdf19a7775a813b4b896178aa6f861f3