Copter: protect against arming while already armed

Previously it was possible to arm the vehicle (from the GCS) even thought the vehicle was already armed which would lead to the motors stopping for 2 seconds

ArduCopter/motors.cpp

@@ -130,6 +130,11 @@ bool Copter::init_arm_motors(bool arming_from_gcs)
     }
     in_arm_motors = true;
 
+    // return true if already armed
+    if (motors.armed()) {
+        return true;
+    }
+
     // run pre-arm-checks and display failures
     if (!all_arming_checks_passing(arming_from_gcs)) {
         AP_Notify::events.arming_failed = true;