* Integrated preliminary ICM20948 support into MPU9250 driver.
Fixed temperature conversion for MPU9250/ICM20948.
* Included missing check for PX4_I2C_OBDEV_MPU9250 in main.cpp.
* Added explicit bus for internal MPU9250 on Pixhawk 2.1 to avoid implicit start
of an externally attached device with wrong orientation.