U-Boot loader for QEMU RISC-V 64-bit

To use this bootloader to boot a FreeBSD RISC-V image, install
emulators/qemu50 and sysutils/opensbi and execute it as:

qemu-system-riscv64 -machine virt -m 2048m -smp 2 -nographic \
    -bios /usr/local/share/opensbi/lp64/generic/firmware/fw_jump.elf \
    -kernel /usr/local/share/u-boot/u-boot-qemu-riscv64/u-boot.bin \
    -drive file=/path/to/freebsd.img,format=raw,id=hd0 \
    -device virtio-blk-device,drive=hd0