CAN support for point-to-point UAVTalk stream

This provides a basic implementation of PIOS_CAN with a PIOS_COM interface.  Currently it uses a hardcoded message ID to indicate this type (and no hardware filtering).  There is a test application which forwards CameraDesired messages via the CAN bus.  At the receiving end, any UAVTalk messages are parsed and unpacked.

This is just the first step to a larger CAN interface being discussed here #580

This has been tested here https://vimeo.com/67528638