U-Boot loader for riscv bhyve

The loader is configured to boot automatically from the first VirtIO block
device.  Specify

  -o bootrom=$LOCALBASE/share/u-boot/u-boot-bhyve-riscv/u-boot.bin

in bhyve(8) parameters.  Note that this loader is experimental as the riscv
bhyve port is still under development.
