riscv: Resolve full path of the given bios image

At present when "-bios image" is supplied, we just use the straight
path without searching for the configured data directories. Like
"-bios default", we add the same logic so that "-L" actually works.

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>