Add support for receiving fixed length packets to the RF12 driver

This allows use of the RF12 driver to receive data from sources that do
not implement the RF12 protocol. Includes a sample for the EMT7110
energy meter.

This version is based on your second idea from yesterday and uses some trickery with the rxfill "state machine". Not as readable but less intrusive. The crc will be calculated but obviously of no use.