sliptty/start_network.sh: fix with DHCPv6
sliptty/start_network.sh: fix with DHCPv6

The `start_network.sh` script creates a `${TUN}` interface, so
the `${TAP}` variable will always be empty.

This means start_dhcpd() will always fail as the DHCPv6 script is lacking
an interface.

OTAkeys/RIOT,miri64/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,miri64/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OTAkeys/RIOT,yogo1212/RIOT,jasonatran/RIOT,OlegHahm/RIOT,basilfx/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,yogo1212/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,authmillenon/RIOT,basilfx/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OTAkeys/RIOT,ant9000/RIOT,yogo1212/RIOT,yogo1212/RIOT,basilfx/RIOT,miri64/RIOT,basilfx/RIOT,authmillenon/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,yogo1212/RIOT,OTAkeys/RIOT,kYc0o/RIOT,jasonatran/RIOT,basilfx/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT