reduce nesting in configure.bat

for some inexplicable reason, configure.bat went into an endless loop
on win 8.1+ while attempting to parse the command line; this is clearly
a bug in cmd, so work around it.

amends 7af6e9bb.

Task-number: QTBUG-58019
Change-Id: I698a2a51891a4e7af75836c075888f70df865409
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

configure.bat

@@ -89,13 +89,12 @@ goto doneargs
 
 :help
     type %QTSRC%\config_help.txt
-    if %TOPLEVEL% == true (
-        for /d %%p in ("%TOPQTSRC%"\qt*) do (
-            if not "%%p" == "%QTSRC%" (
-                if exist "%%p\config_help.txt" (
-                    echo.
-                    type "%%p\config_help.txt"
-                )
+    if %TOPLEVEL% == false exit /b 1
+    for /d %%p in ("%TOPQTSRC%"\qt*) do (
+        if not "%%p" == "%QTSRC%" (
+            if exist "%%p\config_help.txt" (
+                echo.
+                type "%%p\config_help.txt"
             )
         )
     )