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
fix scorpion build

Change-Id: If746970ce5b97fccfe52bd8603a1902f89d9389e
Merge branch 'android-4.4' of https://github.com/omnirom/android_build into android-4.4