Add a tapas command.

(Actually there was a tapas command that just called choosecombo).

The new better tapas command is for building unbundled apps.  Run
it with one or more modules to build and optionally a build variant.

tapas [variant] App1 App2 ...

If you don't supply a build variant, it defaults to eng.

Change-Id: I02214abd0b5ad02e364fcb024e10cf6ad17a9e68