AP_OpticalFlow: move bus definition to AP_HAL header

libraries/AP_HAL/AP_HAL_Boards.h

@@ -431,6 +431,10 @@
 #define HAL_OPTFLOW_PX4FLOW_I2C_ADDRESS 0x42
 #endif
 
+#ifndef HAL_OPTFLOW_PX4FLOW_I2C_BUS
+#define HAL_OPTFLOW_PX4FLOW_I2C_BUS 1
+#endif
+
 #elif CONFIG_HAL_BOARD == HAL_BOARD_EMPTY
 #define HAL_BOARD_NAME "EMPTY"
 #define HAL_CPU_CLASS HAL_CPU_CLASS_16

libraries/AP_OpticalFlow/OpticalFlow.cpp

@@ -51,7 +51,7 @@ OpticalFlow::OpticalFlow(AP_AHRS_NavEKF& ahrs) :
       CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_BBBMINI
     backend(new AP_OpticalFlow_Onboard(*this, ahrs)),
 #elif CONFIG_HAL_BOARD == HAL_BOARD_LINUX
-    backend(new AP_OpticalFlow_Linux(*this, hal.i2c_mgr->get_device(1, HAL_OPTFLOW_PX4FLOW_I2C_ADDRESS))),
+    backend(new AP_OpticalFlow_Linux(*this, hal.i2c_mgr->get_device(HAL_OPTFLOW_PX4FLOW_I2C_BUS, HAL_OPTFLOW_PX4FLOW_I2C_ADDRESS))),
 #else
     backend(NULL),
 #endif