populate_sdk_ext: use ConfigParser instead of SafeConfigParser

SafeConfigParser was renamed to ConfigParser in 3.2, and the
SafeConfigParser alias will be removed in 3.12.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Cherry-picked from master: 71b3e7f7

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>