dextrose-updater 5.4: use networkmanager hooks instead of cron

Cron is not installed in 13.2.0 images anymore.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
dextrose-updater 5.5: add proxy support

Use the proxy value used by the olpc user

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>