lualoader: Execute menu_timeout_command at the end of menu autoboot

Instead of hardcoding "boot", respect menu_timeout_command from Forth. It
still defaults to 'boot', so this will not be a functional change for most.

stand/lua/menu.lua

@@ -448,8 +448,9 @@ function menu.autoboot()
 
 		loader.delay(50000)
 	until time <= 0
-	core.boot()
 
+	local cmd = loader.getenv("menu_timeout_command") or "boot"
+	loader.interpret(cmd)
 end
 
 return menu