This allows backends to have a separate detection and initialization
logic. It doesn't change any backend yet and with the current code
there's no change in behavior either. This only allows
AP_InertialSensor::_detect_backend() to be called earlier so
AP_InertialSensor object can be used by other libraries. If it's not
called, later on AP_InertialSensor::init() will detect and start all
backends.
The calibration on LSM9DS0 was giving offsets between 4.0 and 4.2 on x-axis and
around 3.6 on y-axis. It turned out that those offsets were actually right.
The maximum absolute values of calibration offset should be a sensor
characteristic rather than a constant value for all sensors.
The constant value previously used (3.5 m/s/s for all axes) is set here as a
default maximum absolute calibration offset for every instance to keep it
working.
This converts the MPU6000 driver to a frontend/backend structure, and
disables all other drivers. They will be progressively re-enabled as
each is converted
