CHerry-pick to re-implement airspeed
Airspeed: implement pressure scaling factor.

- Scaling factor implemented for ETS and MEAS/Honeywell sensors
- Allows any sensors in the same family (with different scales) to be used
- Startup/default values are right for the standard sensors
Airspeed compiles, untested.
Dropped some of the warnx statements in hmc5883.cpp. No idea why they don't work, but it's happy now.
Merge branch 'airspeed_sensor_scale_v2' of https://github.com/Slatye/PX4Firmware into airspeed_sensor_scale_v2