Copter: remove unused Log_Write_IMU function
Copter: add DCM_CHECK_THRESH parameter
Copter: add DCM check of yaw error

Triggers an "ekf" failsafe if the DCM yaw error is > 60deg
Copter: enable Optflow by default for Pixhawk only
HAL_AVR: fixed warning
AP_InertialSensor: first steps in frontend/backend split

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
AP_InertialSensor: converted HIL backend, which gets SITL working
SITL: update for new AP_InertialSensor API
AP_InertialSensor: converted PX4 driver to new API
AP_InertialSensor: converted the MPU9250 driver to new API
AP_InertialSensor: converted the APM1/Oilpan driver to new API
AP_InertialSensor: minor tidy up for HIL driver
AP_InertialSensor: improved timing in all drivers
AP_InertialSensor: ported L3G4200D driver to new API
AP_InertialSensor: tidy up MPU6000 driver
AP_InertialSensor: use common AP_InertialSensor driver for PX4 and VRBRAIN
AP_InertialSensor: converted flymaple driver to new API
AP_InertialSensor: converted MPU9150 driver

untested conversion
Plane: update for new AP_InertialSensor API
Copter: convert to new AP_InertialSensor API