net: NC-SI setup and handling

Add the handling of NC-SI ethernet frames, and add a check at the start
of net_loop() to configure NC-SI before starting other network commands.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>