Adds a customizable screen timeout for the default boot screen

Marlin/Conditionals.h

@@ -269,6 +269,10 @@
     #endif
   #endif
 
+  #ifndef BOOTSCREEN_TIMEOUT
+    #define BOOTSCREEN_TIMEOUT 2500
+  #endif
+
 #else // CONFIGURATION_LCD
 
   #define CONDITIONALS_H

Marlin/Marlin_main.cpp

@@ -946,7 +946,7 @@ void setup() {
   lcd_init();
   #if ENABLED(SHOW_BOOTSCREEN)
     #if ENABLED(DOGLCD)
-      delay(1000);
+      delay(BOOTSCREEN_TIMEOUT);
     #elif ENABLED(ULTRA_LCD)
       bootscreen();
       lcd_init();