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.

Change-Id: I935327252098ee74b34a815a023d2ac6b9a53a30