add pin definition to some motherboards

Marlin/Marlin_main.cpp

       do_blocking_move_to(X_MAX_POS + SLED_DOCKING_OFFSET + offset - 1, current_position[Y_AXIS], current_position[Z_AXIS]);  // Dock sled a bit closer to ensure proper capturing                                                                                                                           
       digitalWrite(SLED_PIN, LOW); // turn off magnet
       do_blocking_move_to(oldXpos, current_position[Y_AXIS], current_position[Z_AXIS]); // return to position before docking
-      do_blocking_move_to(current_position[X_AXIS], current_position[Y_AXIS], current_position[Z_AXIS] - Z_RAISE_AFTER_PROBING);
     } else {
       float oldXpos = current_position[X_AXIS]; // save x position
       float z_loc = current_position[Z_AXIS];

Marlin/pins_MEGATRONICS_3.h

 
 #define LARGE_FLASH        true
 
+
+#ifdef Z_PROBE_SLED
+  #define SLED_PIN         -1
+#endif
+
 // Servo support
 #ifdef NUM_SERVOS
   #define SERVO0_PIN       46 //AUX3-6

Marlin/pins_RAMBO.h

   #endif
 #endif
 
+#ifdef Z_PROBE_SLED
+  #define SLED_PIN         -1
+#endif
+
 #undef X_MS1_PIN
 #undef X_MS2_PIN
 #undef Y_MS1_PIN

Marlin/pins_RAMPS_13.h

   #endif
 #endif
 
+#ifdef Z_PROBE_SLED
+  #define SLED_PIN         -1
+#endif
+
 #ifdef ULTRA_LCD
 
   #ifdef NEWPANEL