Adds support for optionally generating vbmeta.img in merge_builds.

Bug: 137853921
Bug: 138671115
Test: python -m unittest test_common
Test: python -m unittest test_add_img_to_target_files
Test: Ran 'merge_builds --build_vbmeta' for two devices, one with the
vbmeta struct on system.img and another with vbmeta_system.img. Flashed
the regenerated vbmeta.img files on devices, devices boot.

Change-Id: I8d7585c7af468be3d242d8aceeed6d27e6fc6d96