logging: use warning instead warn

The warn method is deprecated. We should use the documented warning instead.

Quoting from the python's official doc:
"""
Note: There is an obsolete method warn which is functionally identical to warning.
      As warn is deprecated, please do not use it - use warning instead.
"""

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>