stand/lua: Clear the screen before prompting for passwords

In the worst case scenario, we have no passwords to prompt for and we end up
just clearing the screen twice before we draw the menu or proceed with boot.

In the best case scenario, we don't try drawing password prompts amidst a
bunch of kernel/module loading.

stand/lua/password.lua

@@ -58,6 +58,7 @@ function password.read()
 end
 
 function password.check()
+	screen.clear();
 	screen.defcursor();
 	-- pwd is optionally supplied if we want to check it
 	local function do_prompt(prompt, pwd)