gnrc_ipv6_nib: consider largest prefix match when deciding if host on-link
gnrc_ipv6_nib: consider largest prefix match when deciding if host on-link

If the fib contains a route to a subnet via another host, it can not
be on-link.

Consider fib entries when deciding whether an address is on-link.
If a route via another host is a stronger match than an on-link
prefix, the address must be off-link.

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