[netdevice] Provide a test mechanism for discarding packets at random

[netdevice] Provide a test mechanism for discarding packets at random

Setting NETDEV_DISCARD_RATE to a non-zero value will cause one in
every NETDEV_DISCARD_RATE packets to be discarded at random on both
the transmit and receive datapaths, allowing the robustness of
upper-layer network protocols to be tested even in simulation
environments that provide wholly reliable packet transmission.

Signed-off-by: Michael Brown <mcb30@ipxe.org>