boot/uboot: introduce BR2_TARGET_UBOOT_FORMAT_DTB

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>