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/pkbuff.h

@@ -14,9 +14,6 @@
 #include <assert.h>
 #include <gpxe/list.h>
 
-struct net_protocol;
-struct ll_protocol;
-
 /**
  * Packet buffer alignment
  *
@@ -42,6 +39,9 @@ struct ll_protocol;
  * This structure is used to represent a network packet within gPXE.
  */
 struct pk_buff {
+	/** List of which this buffer is a member */
+	struct list_head list;
+
 	/** Start of the buffer */
 	void *head;
 	/** Start of data */
@@ -50,14 +50,6 @@ struct pk_buff {
 	void *tail;
 	/** End of the buffer */
         void *end;
-
-	/** List of which this buffer is a member */
-	struct list_head list;
-
-	/** The network-layer protocol */
-	struct net_protocol *net_protocol;
-	/** The link-layer protocol */
-	struct ll_protocol *ll_protocol;
 };
 
 /**