Add MPU9150 mag support and mag autodetection. Add AK8975 mag driver.

Note: not working on the sparky, MPU9150 needs passthough enabling but
when enabled the mag and gyro won't ack on the default addresses.  Needs
further investigation.

src/main/sensors/acceleration.h

@@ -30,7 +30,7 @@ typedef enum AccelSensors {
     ACC_MPU9150 = 8,
     ACC_FAKE = 9,
     ACC_NONE = 10
-} AccelSensors;
+} accelSensor_e;
 
 extern uint8_t accHardware;
 extern sensor_align_e accAlign;