[tcpip] Allow supported address families to be detected at runtime

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

src/include/ipxe/tcpip.h

 	sa_family_t sa_family;
 	/** Fixed header length */
 	size_t header_len;
+	/** Network-layer protocol */
+	struct net_protocol *net_protocol;
 	/**
 	 * Transmit packet
 	 *
 		      struct sockaddr_tcpip *st_dest,
 		      struct net_device *netdev,
 		      uint16_t *trans_csum );
+extern struct tcpip_net_protocol * tcpip_net_protocol ( sa_family_t sa_family );
 extern struct net_device * tcpip_netdev ( struct sockaddr_tcpip *st_dest );
 extern size_t tcpip_mtu ( struct sockaddr_tcpip *st_dest );
 extern uint16_t generic_tcpip_continue_chksum ( uint16_t partial,

src/net/ipv4.c

 	.name = "IPv4",
 	.sa_family = AF_INET,
 	.header_len = sizeof ( struct iphdr ),
+	.net_protocol = &ipv4_protocol,
 	.tx = ipv4_tx,
 	.netdev = ipv4_netdev,
 };

src/net/ipv6.c

 	.name = "IPv6",
 	.sa_family = AF_INET6,
 	.header_len = sizeof ( struct ipv6_header ),
+	.net_protocol = &ipv6_protocol,
 	.tx = ipv6_tx,
 	.netdev = ipv6_netdev,
 };

src/net/tcpip.c

 /**
  * Find TCP/IP network-layer protocol
  *
- * @v st_dest		Destination address
+ * @v sa_family		Address family
  * @ret tcpip_net	TCP/IP network-layer protocol, or NULL if not found
  */
-static struct tcpip_net_protocol *
-tcpip_net_protocol ( struct sockaddr_tcpip *st_dest ) {
+struct tcpip_net_protocol * tcpip_net_protocol ( sa_family_t sa_family ) {
 	struct tcpip_net_protocol *tcpip_net;
 
 	for_each_table_entry ( tcpip_net, TCPIP_NET_PROTOCOLS ) {
-		if ( tcpip_net->sa_family == st_dest->st_family )
+		if ( tcpip_net->sa_family == sa_family )
 			return tcpip_net;
 	}
 
-	DBG ( "Unrecognised TCP/IP address family %d\n", st_dest->st_family );
+	DBG ( "Unrecognised TCP/IP address family %d\n", sa_family );
 	return NULL;
 }
 
 	struct tcpip_net_protocol *tcpip_net;
 
 	/* Hand off packet to the appropriate network-layer protocol */
-	tcpip_net = tcpip_net_protocol ( st_dest );
+	tcpip_net = tcpip_net_protocol ( st_dest->st_family );
 	if ( tcpip_net ) {
 		DBG ( "TCP/IP sending %s packet\n", tcpip_net->name );
 		return tcpip_net->tx ( iobuf, tcpip_protocol, st_src, st_dest,
 	struct tcpip_net_protocol *tcpip_net;
 
 	/* Hand off to the appropriate network-layer protocol */
-	tcpip_net = tcpip_net_protocol ( st_dest );
+	tcpip_net = tcpip_net_protocol ( st_dest->st_family );
 	if ( tcpip_net )
 		return tcpip_net->netdev ( st_dest );
 
 	size_t mtu;
 
 	/* Find appropriate network-layer protocol */
-	tcpip_net = tcpip_net_protocol ( st_dest );
+	tcpip_net = tcpip_net_protocol ( st_dest->st_family );
 	if ( ! tcpip_net )
 		return 0;