Rework the rf12_config string about jeelib

[JohnOH]

If we are permitted to change the layout of the string then I would also change how the frequency offset is saved. In the RF12demo branch the frequency offset is stored as a 4 bit and also an 8 bit number rather than a 12 bit number. this was to permit the driver to detect when an 'old' style (pre RF12demo V11) of configuration was in the eeprom.

[jcw]

Good idea. I don't think we need to keep backward compat on the EEPROM, assuming the new one will fail the CRC anyway.

It does require some care in actual use: use matching RF12demo and RF12 driver code, i.e. recompile and reflash both and things should work. There has always been a form of coupling between RF12demo and rf12_config().

[JohnOH]

On 30/09/2013 12:52, Jean-Claude
  Wippler wrote:

  Good idea. I don't think we need to keep backward compat on the
    EEPROM, assuming the new one will fail the CRC anyway.

I'm not sure the CRC fail method will work unless we change the CRC
algorithm. 
  It does require some care in actual use: use matching RF12demo
    and RF12 driver code, i.e. recompile and reflash both and things
    should work. There has always been a form of coupling between
    RF12demo and rf12_config().
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[jcw]

I'd like to keep this as as for RF12 and revisit the issue on the RF69 driver.