read_bundle_plist.sh: support inclusion via source

Rework the script a bit to allow use on the command line, and use
in other scripts via source. Updates build_dmg.sh and
generate_omaha_config.sh to use source instead of calling the
script directly.

Change-Id: I8fcd19e72627c9de00b704fefeec59c4ea32f70a
3 files changed