Change git checkout for IFW

Change how IFW git repo is cloned, so that e.g., gerrit
change tags can be used for --ifw_branch

Change-Id: I2109a1607c12d3ffcfc3d0063aef8225900f80bf
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>