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

Conflicts:
	envsetup.sh
add gcd funtion to envsetup.sh

  gcd - cd to any tracked git project from any other
  path in the build tree.

Change-Id: I8fde827dc51ac4006030712239f9ee6593b07c4c