Support for non Raspberry Pi machines

Add support for building and running on any linux machine, not just
Raspberry Pi hardware.  Since the relay hardware can't exist on a PC,
no valid data will be received or logged.  However, this allows work
on the UI and interface APIs.

  * Upgrade libbcm2835 to v. 1.33
  * libbcm2835: Skip SPI functions in debug mode
  * Use libbcm2835 debug mode for non RPi hardware
  * Minor updates about V2 HW in README and config.h