AP_MotorsMulticopter: add spool_up_complete accessor function

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -119,6 +119,9 @@ public:
     // get_throttle_limit - throttle limit ratio - for logging purposes only
     float               get_throttle_limit() const { return _throttle_limit; }
 
+    // return true if spool up is complete
+    bool spool_up_complete() const { return _multicopter_flags.spool_mode == THROTTLE_UNLIMITED; }
+
     // var_info for holding Parameter information
     static const struct AP_Param::GroupInfo        var_info[];