add '_' function to builtin dictionary at import time

This allows to import many Beremiz python modules before calling
util.misc.InstallLocalRessources().

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema" URI_location="LOCAL://">
  <TargetType/>
</BeremizRoot>