hw/riscv: Use macros for BIOS image names

hw/riscv: Use macros for BIOS image names

The OpenSBI BIOS image names are used by many RISC-V machines.
Let's define macros for them.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210430071302.1489082-7-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>