Add Virtual LAN support.

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.