Make BOOTSCREEN_TIMEOUT generally available

Marlin/Configuration_adv.h

 // Show the E position (filament used) during printing
 //#define LCD_SHOW_E_TOTAL
 
+#if ENABLED(SHOW_BOOTSCREEN)
+  #define BOOTSCREEN_TIMEOUT 4000        // (ms) Total Duration to display the boot screen(s)
+#endif
+
 #if HAS_GRAPHICAL_LCD && HAS_PRINT_PROGRESS
   //#define PRINT_PROGRESS_SHOW_DECIMALS // Show progress with decimal digits
   //#define SHOW_REMAINING_TIME          // Display estimated time to completion
   //#define DGUS_SERIAL_STATS_RX_BUFFER_OVERRUNS  // Fix Rx overrun situation (Currently only for AVR)
 
   #define DGUS_UPDATE_INTERVAL_MS  500    // (ms) Interval between automatic screen updates
-  #define BOOTSCREEN_TIMEOUT      3000    // (ms) Duration to display the boot screen
 
   #if EITHER(DGUS_LCD_UI_FYSETC, DGUS_LCD_UI_HIPRECY)
     #define DGUS_PRINT_FILENAME           // Display the filename during printing

Marlin/src/lcd/dogm/ultralcd_DOGM.cpp

 
   // Show the Marlin bootscreen, with the u8g loop and delays
   void MarlinUI::show_marlin_bootscreen() {
-    #ifndef BOOTSCREEN_TIMEOUT
-      #define BOOTSCREEN_TIMEOUT 2500
-    #endif
     constexpr uint8_t pages = two_part ? 2 : 1;
     for (uint8_t q = pages; q--;) {
       draw_marlin_bootscreen(q == 0);

Marlin/src/lcd/ultralcd.h

       #endif
 
       #if ENABLED(SHOW_BOOTSCREEN)
+        #ifndef BOOTSCREEN_TIMEOUT
+          #define BOOTSCREEN_TIMEOUT 2500
+        #endif
         static void draw_marlin_bootscreen(const bool line2=false);
         static void show_marlin_bootscreen();
         static void show_bootscreen();