riscv: qemu: enable distro boot

Enable distro boot on the qemu-riscv32/64 boards. Supported boot target
devices are VirtIO and DHCP.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alexander Graf <agraf@suse.de>