Add dev button for firmware update (tap loading bar on loading screen)

Author: Sebazzz

Marlin/src/lcd/extui/lib/dgus_creality/DGUSScreenHandler.cpp

@@ -198,6 +198,10 @@ void DGUSScreenHandler::HandleDevelopmentTestButton(DGUS_VP_Variable &var, void
 
   // Act on it
   switch (button_value) {
+    case VP_DEVELOPMENT_HELPER_BUTTON_ACTION_FIRMWARE_UPDATE:
+      ExtUI::injectCommands_P(PSTR("M997"));
+    break;
+
     case VP_DEVELOPMENT_HELPER_BUTTON_ACTION_TO_MAIN_MENU:
       setstatusmessagePGM(PSTR("Dev action: main menu"));
       GotoScreen(DGUSLCD_SCREEN_MAIN, false);

Marlin/src/lcd/extui/lib/dgus_creality/creality_touch/DGUSDisplayDef.h

@@ -469,5 +469,6 @@ constexpr uint16_t ICON_FWRETRACT_AUTO_ENGAGED = 3;
 
 // Development helper
 constexpr uint16_t VP_DEVELOPMENT_HELPER_BUTTON = 0x22D5;
+constexpr uint16_t VP_DEVELOPMENT_HELPER_BUTTON_ACTION_FIRMWARE_UPDATE = 2;
 constexpr uint16_t VP_DEVELOPMENT_HELPER_BUTTON_ACTION_TO_MAIN_MENU = 4;
 constexpr uint16_t VP_DEVELOPMENT_HELPER_BUTTON_ACTION_RESET_DISPLAY = 8;