Reworked internal clock calibration using external crystal.

The original implementation didn't work very well for me, it was probably never really used (calling is commented out and marked "todo"). Also, this one should occupy less flash space. Implementation is based on Atmel application note AVR055. It is fully tested and working. However, the computed calibration value (in normal room temperature) does not differ much from the one burnt into the MCU in factory, more tests under different coditions (battery voltage, temperature) should be done. If the clock fluctuation with temperature is really noticeable, then maybe running the calibration every hour or so could be a good idea.
First part of RFM code separation/refactoring to make it easier to support other radio types.
More RFM code separation/refactoring to make it easier to support other radio types.
Minor cleanups.
Use macros for RFM fifo control.
Removed confusing preprocessor conditionals for HR25.
Use empty inline function instead of define for calibrate_rco when not used.