Make BOOTSCREEN_TIMEOUT generally available

Marlin/src/lcd/dogm/ultralcd_DOGM.cpp

@@ -219,9 +219,6 @@ bool MarlinUI::detected() { return true; }
 
   // 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

@@ -345,6 +345,9 @@ public:
       #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();