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: I2d438209f8a14949c64baa66485c811da384d20b