Fix for R.D.F.G.S.C. with MEGATRONICS 3 board

The support for reprapworld LCD boke the code for the REPRAP_DISCOUNT_FULL_GRAPHIC_SMART_CONTROLLER. See Full Grapics Display does not work any more after #4408  #4439

Marlin/pins_MEGATRONICS_3.h

@@ -28,7 +28,14 @@
   #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"
+#else
   #define BOARD_NAME       "Megatronics v3.0"
+#endif
+
 #define LARGE_FLASH        true
 
 #if ENABLED(Z_PROBE_SLED)
@@ -45,19 +52,19 @@
 #define X_DIR_PIN          57
 #define X_ENABLE_PIN       59
 #define X_MIN_PIN          37
-#define X_MAX_PIN          40 // put to -1 to disable
+#define X_MAX_PIN          40
 
 #define Y_STEP_PIN         5
 #define Y_DIR_PIN          17
 #define Y_ENABLE_PIN       4
 #define Y_MIN_PIN          41
-#define Y_MAX_PIN          38 // put to -1 to disable
+#define Y_MAX_PIN          38
 
 #define Z_STEP_PIN         16
 #define Z_DIR_PIN          11
 #define Z_ENABLE_PIN       3
 #define Z_MIN_PIN          18
-#define Z_MAX_PIN          19 // put to -1 to disable
+#define Z_MAX_PIN          19
 
 #define E0_STEP_PIN        28
 #define E0_DIR_PIN         27
@@ -104,19 +111,25 @@
   #define TEMP_BED_PIN 14 // ANALOG NUMBERING
 #endif
 
+/**
+ * Controllers and LCDs
+ */
 #define BEEPER_PIN 61
 
-#if ENABLED(DOGLCD)
+#define BTN_EN1 44
+#define BTN_EN2 45
+#define BTN_ENC 33
+
+#define BLEN_C 2
+#define BLEN_B 1
+#define BLEN_A 0
 
-  #if ENABLED(U8GLIB_ST7920)
+#if ENABLED(REPRAPWORLD_GRAPHICAL_LCD)
   #define LCD_PINS_RS     56 // CS chip select / SS chip slave select
   #define LCD_PINS_ENABLE 51 // SID (MOSI)
   #define LCD_PINS_D4     52 // SCK (CLK) clock
   #define SD_DETECT_PIN   35
-  #endif
-
 #else
-
   #define LCD_PINS_RS     32
   #define LCD_PINS_ENABLE 31
   #define LCD_PINS_D4     14
@@ -129,15 +142,10 @@
   #define SHIFT_OUT       34
   #define SHIFT_EN        44
 
-  #define SD_DETECT_PIN 56 // Megatronics v3.1 only
+  #if ENABLED(MEGATRONICS_31)
-
+    #define SD_DETECT_PIN 56
+  #else
+    #define SD_DETECT_PIN -1
   #endif
 
-// Buttons are directly attached using keypad
+#endif
-#define BTN_EN1 44
-#define BTN_EN2 45
-#define BTN_ENC 33
-
-#define BLEN_C 2
-#define BLEN_B 1
-#define BLEN_A 0