Add a caution to drivers.h

Marlin/src/core/drivers.h

@@ -21,7 +21,10 @@
  */
 #pragma once
 
-#include "../inc/MarlinConfigPre.h"
+//
+// Included by MarlinConfigPre.h ahead of Configuration_adv.h.
+// Don't use #if in this file for anything not defined early!
+//
 
 #define _A4988              0x4988
 #define _A5984              0x5984