wic: use explicit errno import

wic: use explicit errno import

os.errno doesn't work in Python 3.7 and shouldn't have ever worked, so use
import errno explicitly.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster@mvista.com>