CTP can receive message duplicates. The protocol uses the SentCache to
detect duplicates: when CTP sends a message, it updates the cache
record.

CTP does not update the cache when the message is received at the root
(since the root does not forward/send the message further). This
results in message duplicates at the root.

Added SentCache.insert at the root node, before it signals to the upper layer
receving of a new message. This way, message duplicates are also eliminated at
the last hop, with destination at the root.
use channel 26 on all radios
Merge pull request #258 from szte-wsn/rfxxx_def_channel

Use channel 26 on all radios
Replaced timer-based BusyWait with a main clock based one, because timer-based caused infinite loop in long atomic sections
serial stack by @mmaroti Faster and smaller then the current one
Merge pull request #252 from mszczodrak/ctp_root_cache

CTP can receive message duplicates. The protocol uses the SentCache to