hw/riscv: Extend the kernel loading support

hw/riscv: Extend the kernel loading support

Extend the RISC-V kernel loader to support Image and uImage files.
A Linux kernel can now be booted with:

    qemu-system-riscv64 -machine virt -bios fw_jump.bin -kernel Image

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>