gnrc_lorawan: cleanup PHY state machine

[jia200x]

Contribution description

This PR:

• Implements the PHY state machine of GNRC LoRaWAN using an FSM with entry/exit states. This simplifies the logic a lot, makes it more robust and facilitates adding other LoRaWAN classes.
• Cleans up timers and event functions, as those old function interfaces are not needed anymore.

Testing procedure

Run examples/gnrc_lorawan. Make sure it's still possible to join a LoRaWAN network, send and receive data.

Issues/PRs references

None so far, but it will be required for adding class C support (almost there!)

[jia200x]

not a draft anymore

[riot-ci]

Murdock results

:x: FAILED

873adca1b2317ff506045d32155dfe7f8480dd7c gnrc_lorawan: implement PHY layer using FSM

Success	Failures	Total	Runtime
141	0	6933	01m:39s

Artifacts

[Teufelchen1]

Hi :crocodile:

I would be willing to review this. Is it still up to date? Did you "battle test" this code in the time that has passed since opening the PR? I will probably have a lot of questions as I am not that into Lora; when is the next hack'n'ack where we could go through it together?