Use MB() for Megatronics 3.1

Marlin/src/pins/pins.h

   #include "pins_MEGATRONICS.h"       // ATmega2560
 #elif MB(MEGATRONICS_2)
   #include "pins_MEGATRONICS_2.h"     // ATmega2560
-#elif MB(MEGATRONICS_3)
-  #include "pins_MEGATRONICS_3.h"     // ATmega2560
-#elif MB(MEGATRONICS_31)
-  #define MEGATRONICS_31
+#elif MB(MEGATRONICS_3) || MB(MEGATRONICS_31)
   #include "pins_MEGATRONICS_3.h"     // ATmega2560
 #elif MB(RAMBO)
   #include "pins_RAMBO.h"             // ATmega2560

Marlin/src/pins/pins_MEGATRONICS_3.h

  */
 
 /**
- * MegaTronics v3.0 pin assignments
+ * MegaTronics v3.0 / v3.1 pin assignments
  */
 
 #ifndef __AVR_ATmega2560__
   #error "Oops!  Make sure you have 'Arduino Mega' selected from the 'Tools -> Boards' menu."
 #endif
 
-#define MEGATRONICS_31
-
-#if ENABLED(MEGATRONICS_31)
-  #define BOARD_NAME       "Megatronics v3.1"
+#if MB(MEGATRONICS_31)
+  #define BOARD_NAME "Megatronics v3.1"
 #else
-  #define BOARD_NAME       "Megatronics v3.0"
+  #define BOARD_NAME "Megatronics v3.0"
 #endif
 
 //
   #define SHIFT_OUT        34
   #define SHIFT_EN         44
 
-  #if ENABLED(MEGATRONICS_31)
+  #if MB(MEGATRONICS_31)
     #define SD_DETECT_PIN  56
-  #else
-    #define SD_DETECT_PIN  -1
   #endif
 
 #endif