Add Virtual LAN support.

This patch adds support for virtual LAN (VLAN) tagging. VLAN tagging allows
multiple VLANs in a bridged network to share the same physical network link
but maintain isolation:

http://en.wikipedia.org/wiki/IEEE_802.1Q

* grub-core/net/ethernet.c: Add check, get, and set vlan tag id.
* grub-core/net/drivers/ieee1275/ofnet.c: Get vlan tag id from bootargs.
* grub-core/net/arp.c: Add check.
* grub-core/net/ip.c: Likewise.
* include/grub/net/arp.h: Add vlantag attribute.
* include/grub/net/ip.h: Likewise.

grub-core/net/arp.c

@@ -121,8 +121,8 @@ grub_net_arp_send_request (struct grub_net_network_level_interface *inf,
 }
 
 grub_err_t
-grub_net_arp_receive (struct grub_net_buff *nb,
-		      struct grub_net_card *card)
+grub_net_arp_receive (struct grub_net_buff *nb, struct grub_net_card *card,
+                      grub_uint16_t *vlantag)
 {
   struct arphdr *arp_header = (struct arphdr *) nb->data;
   grub_uint8_t *sender_hardware_address;
@@ -157,6 +157,14 @@ grub_net_arp_receive (struct grub_net_buff *nb,
 
   FOR_NET_NETWORK_LEVEL_INTERFACES (inf)
   {
+    /* Verify vlantag id */
+    if (inf->card == card && inf->vlantag != *vlantag)
+      {
+        grub_dprintf ("net", "invalid vlantag! %x != %x\n",
+                      inf->vlantag, *vlantag);
+        break;
+      }
+
     /* Am I the protocol address target? */
     if (grub_net_addr_cmp (&inf->address, &target_addr) == 0
 	&& grub_be_to_cpu16 (arp_header->op) == ARP_REQUEST)