Add update dmgs to dmg build script.

Also adds some simple optional command line args:
- passing no params or passing "all" will build all dmg files.
- passing "webm" will build the webm updater dmg.
- passing "xiph" will build the xiphqt updater dmg.

Change-Id: I1d1e84f605f767ab589ad908baead48d297623c8
1 file changed