Remove the concept of the media-independent link-layer header and replace

it with metadata in the pkb structure.  This is required since UNDI will
want to be able to parse the link-layer header without destroying it.

src/include/gpxe/pkbuff.h

@@ -26,8 +26,40 @@ struct pk_buff {
 	void *tail;
 	/** End of the buffer */
         void *end;
+
+	/** The network-layer protocol
+	 *
+	 * This is the network-layer protocol expressed as an
+	 * ETH_P_XXX constant, in network-byte order.
+	 */
+	uint16_t net_proto;
+	/** Flags
+	 *
+	 * Filled in only on outgoing packets.  Value is the
+	 * bitwise-OR of zero or more PKB_FL_XXX constants.
+	 */
+	uint8_t flags;
+	/** Network-layer address length 
+	 *
+	 * Filled in only on outgoing packets.
+	 */
+	uint8_t net_addr_len;
+	/** Network-layer address
+	 *
+	 * Filled in only on outgoing packets.
+	 */
+	void *net_addr;
 };
 
+/** Packet is a broadcast packet */
+#define PKB_FL_BROADCAST 0x01
+
+/** Packet is a multicast packet */
+#define PKB_FL_MULTICAST 0x02
+
+/** Network-layer address is a raw hardware address */
+#define PKB_FL_RAW_NET_ADDR 0x04
+
 /**
  * Add data to start of packet buffer
  *