Sensor drivers

These are sensor driver improvements / new laser rangefinder kindly contributed by the APM dev community.
true. It works for our current setups (we only either have hmc5883 externally or both internal and external. We have no board with hmc5883 internal only), but maybe we could add code to handle "hmc5883 info" with internal only, detecting that case by dev_int being non-null and dev_ext being null?