examples/gnrc_border_router: use 64-bit prefix with UHCP
examples/gnrc_border_router: use 64-bit prefix with UHCP

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