Basic RAMPS SD_DETECT_PIN override

Marlin/src/pins/pins_RAMPS.h

@@ -436,7 +436,9 @@
       #endif
 
       #define BTN_ENC           35
-      #define SD_DETECT_PIN     49
+      #ifndef SD_DETECT_PIN
+        #define SD_DETECT_PIN   49
+      #endif
       #define KILL_PIN          41
 
       #if ENABLED(BQ_LCD_SMART_CONTROLLER)