Add script to check GPL projects have released SHA

Checks that the source tree to which the VNDK snapshot is being
installed has the sources for all GPL prebuilts by confirming
the git projects have the SHA values recorded in manifest.xml.

Bug: 70603439
Test: python update.py [options]
Test: python check_gpl_license.py [options]
Change-Id: I5b34271cd0dab737187211f52b2fdce6dbd94e2e