bld_ifw_tools: Use branch 1.5 instead of 1.4 by default for IFW

Change-Id: I167e3e4419b472215587baa6a51a680df5b79aa8
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>