tool/run/image/uboot: provide dtb for FIT images

Our new U-BOOT for the imx8mp_iot_gate complains about the size
of the device tree being zero bytes. This is because it no longer
provides the kernel boot arguments via ATAGS, but passes them via
by injecting them in the flattened device tree..

This commit adds a minimal dtb file to the FIT image to satisfy U-BOOT.

Issue  #5354

tool/run/image/uboot

@@ -36,6 +36,7 @@ proc image_uboot_gzip_opt { } {
 # Build U-boot bootloader specific uImage
 #
 proc run_image { } {
+	set dtc [installed_command dtc]
 
 	set elf_img [file join [run_dir] boot [kernel_specific_binary image.elf]]
 
@@ -67,8 +68,18 @@ proc run_image { } {
 	if {[image_uboot_use_fit]} {
 		# create image.itb
 		set uboot_img [file join [run_dir] boot image.itb]
+
+		# create dummy dtb for version of u-boot requiring it in the fit image
+		set fd [open [run_dir]/dummy.dts w]
+		puts $fd "/dts-v1/;\n / {};"
+		close $fd
+		exec $dtc [run_dir]/dummy.dts -o [run_dir]/dummy.dtb
+
 		exec mkimage -f auto -A $arch -O linux -T kernel -C $compress_type -a $load_addr \
-		             -e $entrypoint -d $bin_img$bin_ext $uboot_img
+		             -e $entrypoint -b [run_dir]/dummy.dtb  -d $bin_img$bin_ext $uboot_img
+
+		# cleanup dummy files
+		file delete -force [run_dir]/dummy.dts [run_dir]/dummy.dtb
 	} else {
 		# create uImage
 		set uboot_img [file join [run_dir] boot uImage]