This is an arch independent SLIP netif. The specific serial hooks must be provided by another file. They are sio_open, sio_read/sio_tryread and sio_send

Usage: This netif can be used in three ways:
1) For NO_SYS==0, an RX thread can be used which blocks on sio_read() until data is received.
2) In your main loop, call slipif_poll() to check for new RX bytes, completed packets are fed into netif->input().
3) Call slipif_received_byte[s]() from your serial RX ISR and slipif_process_rxqueue() from your main loop. ISR level decodes packets and puts completed packets on a queue which is fed into the stack from the main loop (needs SYS_LIGHTWEIGHT_PROT for pbuf_alloc to work on ISR level!).