Makefile: Add (default) DEVICE_TREE to SPL FIT

Makefile: Add (default) DEVICE_TREE to SPL FIT

U-boot allows the default device tree to be overridden from
the build environment using the DEVICE_TREE variable.

Make sure that we include it in the SPL FIT mkimage build step.

This also fixes a broken image in case CONFIG_OF_LIST and
CONFIG_OF_OVERLAY_LIST are unset (i.e., expected to be supplied
by the DEVICE_TREE env var).

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>