Merge pull request prusa3d#3474 from leptun/MK3_xflash_upload_screen

XFlash upload message

Author: leptun

Firmware/optiboot_xflash.cpp

@@ -8,6 +8,7 @@
 #include "stk500.h"
 #include "bootapp.h"
 #include <avr/wdt.h>
+#include "lcd.h"
 
 #define OPTIBOOT_MAJVER 6
 #define OPTIBOOT_CUSTOMVER 0
@@ -159,6 +160,9 @@ uint8_t optiboot_xflash_enter()
   xflash_init();
   wdt_disable();
 
+  lcd_clear();
+  lcd_puts_at_P(0, 1, PSTR(" Upgrading xflash\n Do not disconnect!"));
+
   /* Forever loop: exits by causing WDT reset */
   for (;;) {
     /* get character from UART */