Significantly improves throughput and latency on FRDM-KW41Z.
Ping with a packet of size 100 yields a 30 ms roundtrip with this
patch, 177 ms without. Size 1000 ping before: 1160 ms, after: 188 ms
Add ethos support to use a tcp socket instead of a TTY.
Use 'tcp:host' as serial argument.
Port can be given in place of the baudrate argument.
Default port is 20000 which is used in on IoT-LAB.
