Add OBB flags to support overlays

* Add flags field in OBB footer to support overlays.

* Remove unused 'crypto' and 'filesystem' fields in obbtool (could
  later be supported in the "flags" field of the OBB footer).

* Add notes to document OBB classes before shipping.

Change-Id: I386b43c32c5edef55210acb5d3322639c08010ba