lualoader: only clear the screen before first password prompt

This was previously an unconditional screen clear, regardless of whether or
not we would be prompting for any passwords. This is pointless, given that
the screen clear is only there to put our screen into a consistent state
before we draw the prompts and do cursor manipulation.

This is also the only screen clear besides that to draw the menu.  One can
now see early pre-loader and loader output with the menu disabled, which may
be useful for diagnostics.

Reported by:	ian
MFC after:	3 days

stand/lua/password.lua

@@ -38,6 +38,7 @@ local INCORRECT_PASSWORD = "loader: incorrect password"
 -- Asterisks as a password mask
 local show_password_mask = false
 local twiddle_chars = {"/", "-", "\\", "|"}
+local screen_setup = false
 
 -- Module exports
 function password.read(prompt_length)
@@ -80,8 +81,6 @@ function password.read(prompt_length)
 end
 
 function password.check()
-	screen.clear()
-	screen.defcursor()
 	-- pwd is optionally supplied if we want to check it
 	local function doPrompt(prompt, pwd)
 		local attempts = 1
@@ -90,6 +89,12 @@ function password.check()
 			printc("\r" .. string.rep(" ", #INCORRECT_PASSWORD))
 		end
 
+		if not screen_setup then
+			screen.clear()
+			screen.defcursor()
+			screen_setup = true
+		end
+
 		while true do
 			if attempts > 1 then
 				clear_incorrect_text_prompt()