Base work on RFM69 modules

[dewi-ny-je]

RFM12 modules are getting more difficult to find.

Basing this project on a RFM69 module used in RFM12-compatible mode would be preferred.

Porting the whole OpenHR20 to RFM69 to reduce power consumption and increase range would be even better, but it involves a lot more work and it's beyond the scope of this project.

[volca02]

I'm not familiar with RFM69 enough to judge what changes to the protocol would be needed - if any. If there's a RFM12 compatible mode in RFM69, it should be possible to implement the support.

Currently the biggest problem of this project is timing and memory - interrupt based communication with RFM would be beneficial to current polling methods - polling works but places big constraints on the rest of the code (namely WiFi communication).

Honestly I need to get back to the project and finish it to at least reliably working state as is. Base communication works well, but the stability leaves something to desire.