This moves the following parts of ethos' state machine out of ISR
context:
- Sending and replying to HELLO messages
- Byte-unstuffing
Some escape handling is still needed in the ISR handler, due to ethos'
protocol design, to determine if a received byte must go into the
netdev queue (tsrb) or the STDIO queue (isrpipe), but the actual
unstuffing is now done in the STDIO and netdev handler threads,
respectively.
