gnrc_ipv6_nib: set NCE stale on incoming ARO
gnrc_ipv6_nib: set NCE stale on incoming ARO

In accordance with RFC 6775, section 5.2 an NCE should be set STALE
when an ARO renews the address registration for the address:

> The routers SHOULD NOT garbage-collect Registered NCEs (see
> Section 3.4), since they need to retain them until the Registration
> Lifetime expires.  Similarly, if NUD on the router determines that
> the host is UNREACHABLE (based on the logic in [RFC4861]), the NCE
> SHOULD NOT be deleted but rather retained until the Registration
> Lifetime expires.  A renewed ARO should mark the cache entry as
> STALE.  Thus, for 6LoWPAN routers, the Neighbor Cache doesn't behave
> like a cache.  Instead, it behaves as a registry of all the host
> addresses that are attached to the router.

kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT