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: NKlaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>

Makefile

@@ -1386,6 +1386,7 @@ MKIMAGEFLAGS_u-boot.img = -f auto -A $(ARCH) -T firmware -C none -O u-boot \
 	-a $(CONFIG_SYS_TEXT_BASE) -e $(CONFIG_SYS_UBOOT_START) \
 	-p $(CONFIG_FIT_EXTERNAL_OFFSET) \
 	-n "U-Boot $(UBOOTRELEASE) for $(BOARD) board" -E \
+	$(patsubst %,-b arch/$(ARCH)/dts/%.dtb,$(subst ",,$(DEVICE_TREE))) \
 	$(patsubst %,-b arch/$(ARCH)/dts/%.dtb,$(subst ",,$(CONFIG_OF_LIST))) \
 	$(patsubst %,-b arch/$(ARCH)/dts/%.dtbo,$(subst ",,$(CONFIG_OF_OVERLAY_LIST)))
 else