Fix the warning:

E:\projects\rt-thread\rtthread-svn\trunk\tools\keil.py:72: FutureWarning: The behavior of this method will change in future versions.  Use specific 'len(elem)' or 'elem is not None' test instead.
  if not groups:

See http://docs.python.org/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.remove for detail.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2226 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1970 bbd45198-f89e-11dd-88c7-29a3b14d5316