Feature/observer pattern for rx message call back

[jmvdwerf]

All rx messages available in the code have been rewritten into the observer pattern. Only check I could do remotely is a check whether it compiles. At my machine it does. Hence, the big question, does it also run on the chip with the expected behavior? Also, I found two functions that are called, but which I cannot find in the code:

• loconet_rx_prog_task_final
• loconet_rx_prog_task_start

Please have a look at those as well...

The component fast_clock is also a candidate to implement a separate observer, as it also has an update function that is weakly referenced. For now, I just updated the RX part.

[fuegas]

These were defined in loconet/loconet_rx.c:

LOCONET_RX_DUMMY_N(prog_task_start);
LOCONET_RX_DUMMY_N(prog_task_final);