buildchroot: remove FutureWarning printed by etree.py

/home/local/projects/elbe/elbe/elbepack/treeutils.py:69: FutureWarning: This search is broken in 1.3 and earlier, and will be fixed in a future version.  If you rely on the current behaviour, change it to './project/mirror/url-list'
  return not (self.et.find(path) is None)

Signed-off-by: Manuel Traut <manut@linutronix.de>