string.rstrip() doesn't exist in Python 3, so use the .rstrip method on the
object itself instead.
(From OE-Core rev: 554719e3f65e468f89a96d4869766d3ff7a8012e)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>