boot/uboot: introduce BR2_TARGET_UBOOT_FORMAT_DTB

If U-Boot CONFIG_OF_SEPARATE is defined [1], then the device tree will
be built and placed in a u-boot.dtb file alongside u-boot.bin.

When BR2_TARGET_UBOOT_FORMAT_DTB is enabled, buildroot will copy
u-boot.dtb to $(BINARIES_DIR).

This is useful for RISC-V platforms that want to build OpenSBI with an
external DTB by using FW_FDT_PATH [2].

[1] https://github.com/u-boot/u-boot/blob/master/doc/README.fdt-control
[2] https://github.com/riscv/opensbi/blob/master/docs/firmware/fw.md

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/uboot/Config.in

@@ -252,6 +252,9 @@ config BR2_TARGET_UBOOT_FORMAT_BIN
 	bool "u-boot.bin"
 	default y
 
+config BR2_TARGET_UBOOT_FORMAT_DTB
+	bool "u-boot.dtb"
+
 config BR2_TARGET_UBOOT_FORMAT_DTB_BIN
 	bool "u-boot-dtb.bin"
 

boot/uboot/uboot.mk

@@ -48,6 +48,10 @@ ifeq ($(BR2_TARGET_UBOOT_FORMAT_BIN),y)
 UBOOT_BINS += u-boot.bin
 endif
 
+ifeq ($(BR2_TARGET_UBOOT_FORMAT_DTB),y)
+UBOOT_BINS += u-boot.dtb
+endif
+
 ifeq ($(BR2_TARGET_UBOOT_FORMAT_ELF),y)
 UBOOT_BINS += u-boot
 # To make elf usable for debuging on ARC use special target