net/net: Add net_set_vlan command

net/net: Add net_set_vlan command

Previously there was no way to set the 802.1Q VLAN identifier, despite
support for vlantag in the net module. The only location vlantag was
being populated was from PXE boot and only for Open Firmware hardware.
This commit allows users to manually configure VLAN information for any
interface.

Example usage:
  grub> net_ls_addr
  efinet1 00:11:22:33:44:55 192.0.2.100
  grub> net_set_vlan efinet1 100
  grub> net_ls_addr
  efinet1 00:11:22:33:44:55 192.0.2.100 vlan100
  grub> net_set_vlan efinet1 0
  efinet1 00:11:22:33:44:55 192.0.2.100

Signed-off-by: Chad Kimes <chkimes@github.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>