HAL_AVR: fixed warning
Plane: disable CLI by default

it really isn't needed any more, and saves 20k of flash on APM2
Plane: display min as well as max loop delta_t
AP_L1_Control: fixed some warnings
AP_TECS: fixed some warnings
AP_Terrain: fixed a 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