This commit implements uplink redundancy in GNRC LoRaWAN. Uplink
redundancy is used to retransmit unconfirmed uplink frames. The
retransmission stops when either a downlink message is received or the
number of uplink retransmissions is reached.
This functionality doesn't affect confirmed uplinks.
This commit removes the dependency to xtimer and RIOT messages.
This step is required to use other sources of events (e.g event_queue)
and timers (RTT)
