more work on RR-NET emulation.
implement more of the ethernet TX path for RR-NET emulation.
buffer ethernet frame bytes via RR-NET emulation registers.
actually send ethernet frame when indicated by RR-NET emulation
registers.
add extra RX buffer for RR-NET emulation and plumb in.
All RR-NET machinery should now be available.
fix perpetually mapped rr-net ethernet buffer.
revert to test bench sending a real ping packet.