LWIP_ND6_RDNSS_MAX_DNS_SERVERS and ipv6

[nuclearcat]

Please make it enabled, or (better) as configurable option for IPv6 setup (to enable it need to be set to 1 at least), as there is no other way at moment to receive ipv6 DNS server.
I edited components/lwip/port/esp8266/include/lwipopts.h manually and compared code size of final project_template.bin, difference is only 384 bytes and when enabled, set to 1, DNS requests over ipv6 started to work fine.

[donghengqaz]

We have added the configuration to menuconfig, please check.