For mqtt-sn host:
-> subscribes for topics
-> handle publish message
TODO:
-> attach topic id for each topic to subscribes
-> following the topic id of publish message received, handle the
publish message. For the moment when we send "yellow" with any topic id
the yellow led light up. Normally the yellow led will light up only if the
the topic id is for led.


For others files:
-> add test of broadcast example
-> add mcast6 example
-> add uip-mcast6 in core/net/
-> add some files in tmp folder (maybe usefull later)