configure.bat: Fix call without arguments

If there are no arguments, echo will print its status, and config.opt.in
will contain

  ECHO
  is
  off.

Fix this by using "echo." idiom.

This amends 57ad532e75

Pick-to: 6.2 6.3
Change-Id: I1697e59f23cd28256a4a6ce357eae70e36c49d5a
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

configure.bat

@@ -104,7 +104,7 @@ cd "%TOPQTDIR%"
 
 rem Write config.opt if we're not currently -redo'ing
 if "!rargs!" == "" (
-    echo %*>config.opt.in
+    echo.%*>config.opt.in
     cmake -DIN_FILE=config.opt.in -DOUT_FILE=config.opt -DIGNORE_ARGS=-top-level -P "%QTSRC%\cmake\QtWriteArgsFile.cmake"
 )