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 allows us to detect if accel calibration was done in sensor frame
or not. If it was done in sensor frame then the accel calibration is
independent of AHRS_ORIENTATION, which makes it easier to move a board
to a new airframe without having to recalibrate.
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