fix(dns): Fully enable IPv6 by configuring DNS (both DHCPv6 Stateless and RDNSS) (espressif#166)

There are two main IPv6 DNS systems, DHCPv6 Stateless and RDNSS, used in different networks; this change enables both of them.

Previously IPv6 was enabled and assigned addresses, via autoconfig Router Advertisement (RA), but did not enable DNS, without which it cannot connect via host names on an IPv6-only network, so was not very useful.

There were some mitigations, e.g. you could statitically configure DNS, or if running in a dual-stack network then the IPv4 DNS will return results (including IPv6 addresses), however applications should automatically work across all network types, so the options in this change should be enabled.

Author: sgryphon

configs/defconfig.common

@@ -81,6 +81,8 @@ CONFIG_SPI_FLASH_WRITE_CHUNK_SIZE=4096
 CONFIG_SPIRAM_MALLOC_ALWAYSINTERNAL=4096
 CONFIG_SPIRAM_MALLOC_RESERVE_INTERNAL=0
 CONFIG_LWIP_IPV6_AUTOCONFIG=y
+CONFIG_LWIP_IPV6_DHCP6=y
+CONFIG_LWIP_IPV6_RDNSS_MAX_DNS_SERVERS=2
 CONFIG_ESP_RMAKER_SKIP_VERSION_CHECK=y
 CONFIG_ESP_RMAKER_USER_ID_CHECK=y
 CONFIG_ESP_INSIGHTS_ENABLED=y