Revert "Tools: update reltopdir function to use relpath()"

This reverts commit 392911fc62.

Tools/autotest/pysim/util.py

@@ -63,7 +63,7 @@ def relcurdir(path):
 
 def reltopdir(path):
     """Return a path relative to topdir()."""
-    return os.path.relpath(path, topdir())
+    return os.path.normpath(os.path.join(topdir(), path))
 
 
 def run_cmd(cmd, directory=".", show=True, output=False, checkfail=True):