[ipv6] Replace IPv6 stack

Replace the existing partially-implemented IPv6 stack with a fresh
implementation.

This implementation is not yet complete.  The IPv6 transmit and
receive datapaths are functional (including fragment reassembly and
parsing of arbitrary extension headers).  NDP neighbour solicitations
and advertisements are supported.  ICMPv6 echo is supported.

At present, only link-local addresses may be used, and there is no way
to specify an IPv6 address as part of a URI (either directly or via
a DNS lookup).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/config.c

@@ -101,6 +101,9 @@ REQUIRE_OBJECT ( debugcon );
 #ifdef NET_PROTO_IPV4
 REQUIRE_OBJECT ( ipv4 );
 #endif
+#ifdef NET_PROTO_IPV6
+REQUIRE_OBJECT ( ipv6 );
+#endif
 
 /*
  * Drag in all requested PXE support

src/config/config_route.c

@@ -22,3 +22,6 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #ifdef NET_PROTO_IPV4
 REQUIRE_OBJECT ( route_ipv4 );
 #endif
+#ifdef NET_PROTO_IPV6
+REQUIRE_OBJECT ( route_ipv6 );
+#endif

src/config/general.h

@@ -40,6 +40,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
  */
 
 #define	NET_PROTO_IPV4		/* IPv4 protocol */
+#undef	NET_PROTO_IPV6		/* IPv6 protocol */
 #undef	NET_PROTO_FCOE		/* Fibre Channel over Ethernet protocol */
 
 /*