Copter: position_ok when optical flow ok

Previously the GPS based absolute position was required
This allows using optical flow in all flight modes

ArduCopter/control_brake.cpp

@@ -9,7 +9,7 @@
 // brake_init - initialise brake controller
 bool Copter::brake_init(bool ignore_checks)
 {
-    if (position_ok() || optflow_position_ok() || ignore_checks) {
+    if (position_ok() || ignore_checks) {
 
         // set desired acceleration to zero
         wp_nav.clear_pilot_desired_acceleration();

ArduCopter/control_loiter.cpp

@@ -9,7 +9,7 @@
 // loiter_init - initialise loiter controller
 bool Copter::loiter_init(bool ignore_checks)
 {
-    if (position_ok() || optflow_position_ok() || ignore_checks) {
+    if (position_ok() || ignore_checks) {
 
         // set target to current position
         wp_nav.init_loiter_target();

ArduCopter/system.cpp

@@ -329,7 +329,7 @@ bool Copter::position_ok()
     }
 
     // check ekf position estimate
-    return ekf_position_ok();
+    return (ekf_position_ok() || optflow_position_ok());
 }
 
 // ekf_position_ok - returns true if the ekf claims it's horizontal absolute position estimate is ok and home position is set