Add update installers to installer build script.

Also adds some simple optional command line args:
- passing no params or passing "all" will build all installers
  and show the warning about mpkg builds.
- passing "webm" will build the webm installer and updater
  packages
- passing "xiph" will build the xiphqt installer and updater
  packages.

Change-Id: Id5c970fcadd0d4c304c4493c31205c3de851b95c