Adding initial merge_target_files.py script to merge two target files packages.

This script takes as input two partial target files (one contains system bits,
and the other contains non-system, or other, bits). The script merges the
contents of the two partial target files packages to produce a complete target
files package.

Bug: 123430711
Test: Build two partial target files, merge, compare with full target files.
Test: Validate merged target files via validate_target_files.py.
Change-Id: Ic24acf43b86fc703fb4c970688b006291a1861f8