Simplify RX data path.

Kill off the static single net device and move to proper dynamic
registration (which we need with the new device model).

Break the (flawed) assumption that all network-layer protocols can use
ARP; such network-layer protocols (i.e. IPv4) must now register as an ARP
protocol using ARP_NET_PROTOCOL() and provide a single method for checking
the existence of a local network-layer address.

src/include/gpxe/arp.h

@@ -7,9 +7,33 @@
  *
  */
 
+#include <gpxe/tables.h>
+
 struct net_device;
 struct net_protocol;
 
+/** A network-layer protocol that relies upon ARP */
+struct arp_net_protocol {
+	/** Network-layer protocol */
+	struct net_protocol *net_protocol;
+	/** Check existence of address
+	 *
+	 * @v netdev	Network device
+	 * @v net_addr	Network-layer address
+	 * @ret rc	Return status code
+	 */
+	int ( * check ) ( struct net_device *netdev,
+			  const void *net_addr );
+};
+
+/**
+ * Register an ARP protocol
+ *
+ * @v protocol		ARP protocol
+ */
+#define ARP_NET_PROTOCOL( protocol ) \
+	struct arp_net_protocol protocol __table ( arp_net_protocols, 01 )
+
 extern int arp_resolve ( struct net_device *netdev,
 			 struct net_protocol *net_protocol,
 			 const void *dest_net_addr,