Build: Add support for specifying build variant in brunch/breakfast

This little modification allows specifying build variant in brunch/breakfast commands. For example we can use "brunch i9300 user" to build user variant instead of default userdebug.

When no extra argument is given, userdebug is default.
Allow prebuild APK's in Vendor

Change-Id: I361c32fac1e003ec985ffe2282bf8ef922b11ac0