Added pthread cancelling when calling opticflow_module_stop()

Added pthread cancelling of opticflow thread and returns error on failure.

closes #1578

sw/airborne/modules/computer_vision/opticflow_module.c

@@ -213,7 +213,10 @@ void opticflow_module_stop(void)
   // Stop the capturing
   v4l2_stop_capture(opticflow_dev);
 
-  // TODO: fix thread stop
+  // Cancel the opticalflow calculation thread
+  if (pthread_cancel(opticflow_calc_thread) != 0) {
+    printf("Thread cancel did not work\n");
+  }
 }
 
 /**