Add support for loading a firmware file for the virt machine and the
SiFive U. This can be run with the following command:

    qemu-system-riscv64 -machine virt -bios fw_jump.bin -kernel vmlinux
Signed-off-by: NAlistair Francis <alistair.francis@wdc.com>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>
Tested-by: NBin Meng <bmeng.cn@gmail.com>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>