riscv: Add a helper routine for finding firmware

This adds a helper routine for finding firmware. It is currently
used only for "-bios default" case.

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