loader_lua: consider userboot console as serial

We use ascii box chars with serial console because we do not know
if terminal can draw unixode box chars. Same problem is about userboot
console.

MFC after: 5 days

stand/lua/core.lua

@@ -403,7 +403,10 @@ end
 function core.isSerialConsole()
 	local c = loader.getenv("console")
 	if c ~= nil then
-		if c:find("comconsole") ~= nil then
+		-- serial console is comconsole, but also userboot.
+		-- userboot is there, because we have no way to know
+		-- if the user terminal can draw unicode box chars or not.
+		if c:find("comconsole") ~= nil or c:find("userboot") ~= nil then
 			return true
 		end
 	end