tapdev: Log to stderr; add tracing
tapdev: Log to stderr; add tracing

Use stderr for logging to avoid polluting stdout.
Add some DEBUG tracing.

MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,MohamedSeliem/contiki,bluerover/6lbr,arurke/contiki,bluerover/6lbr,bluerover/6lbr,arurke/contiki,bluerover/6lbr,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki,bluerover/6lbr,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki