This fixes the initialization for Linux boards using the PX4Flow module.
The problem is that after the conversion to use I2CDevice we now need to
use I2CDeviceManager, which is statically constructed after the vehicle
object.
So, if we try to call hal.i2c_mgr->get_device(), it will call the
get_device() method before the constructor is called and receive a
SIGSEGV:
Program received signal SIGSEGV, Segmentation fault.
0x000b06c0 in OpticalFlow::OpticalFlow (this=0x140914 <copter+4980>, ahrs=...)
at ../../libraries/AP_OpticalFlow/OpticalFlow.cpp:54
54 ../../libraries/AP_OpticalFlow/OpticalFlow.cpp: No such file or directory.