Now that all RISC-V machines can use OpenSBI BIN images, we remove
OpenSBI ELF images and also exclude these images from BIOS build.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>