While fixing up pkg.mxe.cc they move the URLs around a bit and dropped
Jessie support in favour of Stretch. We also need to update the keys
used to verify the packages.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>