FMUv2: added define for PX4_I2C_BUS_ONBOARD

needed for hmc5883 on main bus (for FMUv3)
lib/conversion: added rotate_3f()

will be used for user specified rotations in sensor drivers
mpu6000: added -R rotation option
lsm303d: added -R rotation option
l3gd20: added -R rotation option