Driver for the reading Graupner HoTT sensors (such as their ESC range) via the telemetry port

The hott_sensor driver adds support for reading telemetry output from Graupner HoTT sensors, in this case the new range of ESCs for fixedwing aircraft. The sensor's single wire telemetry cable is connector to a free UART on the PX4. This allows the PX4 to read the ESC's RPM, current, temperature and voltage directly.

Tested with the 70A controller (http://www.graupner.de/en/products/fe157a63-1203-43c8-866d-64197ac67366/33770/product.aspx) but there are a number of ESCs ranging from 18A to 150A.

This change also saw the refactoring of the existing hott_telemetry driver so that code could be shared and one run independently of the other. The telemetry driver also sends the ESC data to the transmitter.