qibuild.project: add get_build_dirs
qibuild.project: add get_build_dirs

(used only by qibuild clean -z for now)

Change-Id: I935185659869f4217b3296a94597f5f304067536

aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,aldebaran/qibuild