FlyingF4 fixes for MPU6050

Some minor fixes in pios_board.c for the I2C sensor init (prevents hanging)
Added void PIOS_MPU6050_SetPassThrough(bool passThrough) in pios_mpu6050.c to use the magnetometer as an External I2C sensor.
pios_board.h is configured for a 10DoF GY86 board (MPU6050, HMC5883L and MS5611).
Current issue: see issue 1021 (https://github.com/TauLabs/TauLabs/issues/1021)