Adds a customizable screen timeout for the default boot screen

Marlin/Conditionals.h

     #define LCD_CONTRAST_MIN 60
     #define LCD_CONTRAST_MAX 140
   #endif
-  
+
   #if ENABLED(MAKRPANEL) || ENABLED(MINIPANEL)
     #define DOGLCD
     #define ULTIPANEL
     #endif
   #endif
 
+  #ifndef BOOTSCREEN_TIMEOUT
+    #define BOOTSCREEN_TIMEOUT 2500
+  #endif
+
 #else // CONFIGURATION_LCD
 
   #define CONDITIONALS_H

Marlin/Marlin_main.cpp

   lcd_init();
   #if ENABLED(SHOW_BOOTSCREEN)
     #if ENABLED(DOGLCD)
-      delay(1000);
+      delay(BOOTSCREEN_TIMEOUT);
     #elif ENABLED(ULTRA_LCD)
       bootscreen();
       lcd_init();