px4: Add RTC support

These changes _should_ at least not break things if you don't update
the nuttx and px4 trees.  But to actually return real RTC values
you need to update PX4Firmware and PX4Nuttx with:
https://github.com/diydrones/PX4NuttX/pull/3
https://github.com/diydrones/PX4Firmware/pull/11