Some enhancements to RFM12B driver

[tht]

• faster FIFO reading while receiving
• rough estimate for RSSI using DRSSI bit
• handle irq depending on RFM12B feedback
• Wait for RFM12B power-on-reset before initializing

[jcw]

Thanks - I've merged these into a new branch called "rf12mods", so we can explore and test these and further changes.