lualoader: Invalidate the screen from menu perspective upon mnu exit

In the common case, this will effectively do nothing as the menu will get
redrawn as we leave submenus regardless of whether the screen has been
marked invalid or not

However, upon escape to the loader prompt, one could do either of the
following to re-enter the menu system:

-- Method 1
require('menu').run()

-- Method 2
require('menu').process(menu.default)

With method 1, the menu will get redrawn anyways as we do this before
autoboot checking upon entry. With method 2, however, the menu will not be
redrawn without this invalidation.

Both methods are acceptable for re-entering the menu system, although the
latter method in the local module for processing new and interesting menus
is more expected.

stand/lua/menu.lua

@@ -407,6 +407,10 @@ function menu.process(m, keypress)
 			menu.redraw(m)
 		end
 	end
+	-- Invalidate the screen upon exit so that it gets redrawn upon
+	-- processing a new menu, assuming it won't be redrawn after leaving
+	-- this menu
+	screen_invalid = false
 end
 
 function menu.run()