Currently the make rules are wrongly using qemu/virt opensbi image
for sifive_u machine. Correct it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Chih-Min Chao <chihmin.chao@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1564812484-20385-1-git-send-email-bmeng.cn@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>