Sub: Disable optical flow by default

ArduSub/APM_Config.h

@@ -28,9 +28,9 @@
 #define GRIPPER_ENABLED           DISABLED            // disable gripper to save 500bytes of flash
 //#define CLI_ENABLED           DISABLED            // disable the CLI (command-line-interface) to save 21K of flash space
 //#define NAV_GUIDED            DISABLED            // disable external navigation computer ability to control vehicle through MAV_CMD_NAV_GUIDED mission commands
-//#define OPTFLOW               DISABLED            // disable optical flow sensor to save 5K of flash space
 
 // features below are disabled by default on all boards
+//#define OPTFLOW               ENABLED // enable optical flow sensor support
 //#define DISALLOW_GCS_MODE_CHANGE_DURING_RC_FAILSAFE   // disable mode changes from GCS during Radio failsafes.  Avoids a race condition for vehicle like Solo in which the RC and telemetry travel along the same link
 
 // other settings

ArduSub/config.h

@@ -268,7 +268,7 @@
 //////////////////////////////////////////////////////////////////////////////
 //  OPTICAL_FLOW
 #ifndef OPTFLOW
- # define OPTFLOW       ENABLED
+ # define OPTFLOW       DISABLED
 #endif
 
 //////////////////////////////////////////////////////////////////////////////