riscv: Change the default behavior if no -bios option is specified

riscv: Change the default behavior if no -bios option is specified

Per QEMU deprecated doc, QEMU 4.1 introduced support for the -bios
option in QEMU for RISC-V for the virt machine and sifive_u machine.
The default behavior has been that QEMU does not automatically load
any firmware if no -bios option is included.

Now 2 releases passed, it's time to change the default behavior to
load the default OpenSBI firmware automatically. The firmware is
included with the QEMU release and no user interaction is required.
All a user needs to do is specify the kernel they want to boot with
the -kernel option.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1588335545-649-1-git-send-email-bmeng.cn@gmail.com
Message-Id: <1588335545-649-1-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>