espressif: ipv6: Disable RDNSS

Recursive DNS Server (RDNSS) is an extension of IPv6 Neighbor Discovery.
It is one of several ways (in addition to DHCPv6) to inform a network
node of a usable DNS server.

In LWIP, it appears any RDNSS DNS server will overwrite a DNS server
from DHCPv4 or manual configuration, whether or not CircuitPython has
an IPv6 address configured.

In the (default) case where DHCPv6 is disabled in CircuitPython, but the
(apparently rare) case where a RDNSS advertisement is broadast,
this means that DNS doesn't work, and it appears that assigning to
the dns or ipv4_dns properties of the Radio object doesn't work
(when in reality it's frequently being reset to the value from RDNSS)

On my network, the same DNS server is advertised by DHCPv6, so with this
change  I still get a working v6 configuration with a v6 DNS server
when I enable DHCPv6, but when configuring as v4 only (the default),
the correct IPv4 DNS server setting happens and I can also manually
change it by assigning .dns or .ipv4_dns.

Author: jepler

ports/espressif/esp-idf-config/sdkconfig.defaults

@@ -67,7 +67,7 @@ CONFIG_LWIP_SO_RCVBUF=y
 # IPv6
 #
 CONFIG_LWIP_IPV6_AUTOCONFIG=y
-CONFIG_LWIP_IPV6_RDNSS_MAX_DNS_SERVERS=2
+CONFIG_LWIP_IPV6_RDNSS_MAX_DNS_SERVERS=0
 CONFIG_LWIP_IPV6_DHCP6=y
 #
 # TCP