Run the emulation with:

  qemu-system-riscv32 -M virt -kernel output/images/bbl -append "root=/dev/vda ro console=ttyS0" -drive file=output/images/rootfs.ext2,format=raw,id=hd0 -device virtio-blk-device,drive=hd0 -netdev user,id=net0 -device virtio-net-device,netdev=net0 -nographic

The login prompt will appear in the terminal that started Qemu.

Tested with QEMU 2.12.1