added a partial release option to "m84" aka "stepper release". If you do tiny layers, you might want to keep the z-axis powered to not loose height positioning after homeing.

Marlin/cardreader.pde

@@ -436,7 +436,7 @@ void CardReader::printingHasFinished()
  if(SD_FINISHED_STEPPERRELEASE)
  {
    //finishAndDisableSteppers();
-   enquecommand("M84");
+   enquecommand(SD_FINISHED_RELEASECOMMAND);
  }
  autotempShutdown();
 }