[peripherals][imu] i2c slaves for the mpu60x0

new IMU drivers using the mpu60x0 peripheral with slave support
- aspirin 2 (spi) driver with mag as real slave
- drotek 10dof v2 with mag as passthrough slave (untested and axes might be wrong)

closes #450
closes #190
[rotorcraft] rc euler read in fixed point

closes #462