dnsmasq: add IPv6 nameserver configuration in server mode

When in ra server mode, configure nameservers passed in router
announcements from the dns value (which is already used by odhcpd).

This also fixes FS#677 by using the global IPv6 address of the router
instead of the link local address (if no nameservers are configured).

Signed-off-by: Arjen de Korte <build+lede@de-korte.org>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [PKG_RELEASE increase]