SD detect override for older Creality3D RAMPS (#17469)

Marlin/src/pins/ramps/pins_RAMPS_CREALITY.h

   #define FIL_RUNOUT2_PIN                     15  // Creality CR-X can use dual runout sensors
 #endif
 
-#define SD_DETECT_PIN                         49  // Always define onboard SD detect
+#ifndef SD_DETECT_PIN
+  #define SD_DETECT_PIN                       49  // Always define onboard SD detect
+#endif
 
 #define PS_ON_PIN                             40  // Used by CR2020 Industrial series