rp2/lwip_inc: Enable IPv6 per default on rp2 port.

Having IPv6 support is important, especially for IoT-Devices which might be
many, requiring individual IP-addresses. In particular direct access via
link-local addresses and having deterministic SLAAC-addresses can be quite
convenient. Also in IPv6-only networks or for connecting to IPv6-only
services, this is very useful.

For the Pico W, there is enough flash and RAM that enabling IPv6 by default
is the right choice.

Should IPv6 support in a network exist (i.e. there are Router
Advertisements), but not provide connectivity, connecting by domain name
should not be a problem as DNS will default to return the IPv4-address (if
that exists), unless reconfigured at runtime to prefer IPv6.

In any case a user can disable obtaining SLAAC-addresses with:

    <nic>.ipconfig(autoconf6=False)

Signed-off-by: Felix Dörre <felix@dogcraft.de>

ports/rp2/lwip_inc/lwipopts.h

@@ -26,7 +26,10 @@
 #define LWIP_NETIF_EXT_STATUS_CALLBACK  1
 #define LWIP_NETIF_STATUS_CALLBACK      1
 
-#define LWIP_IPV6                       0
+#define LWIP_IPV4                       1
+#define LWIP_IPV6                       1
+#define LWIP_ND6_NUM_DESTINATIONS       4
+#define LWIP_ND6_QUEUEING               0
 #define LWIP_DHCP                       1
 #define LWIP_DHCP_CHECK_LINK_UP         1
 #define DHCP_DOES_ARP_CHECK             0 // to speed DHCP up