A VLAN tagged DHCP request/discover is 4 bytes short

The problem is that BOOTP_SIZE uses ETHER_HDR_SIZE which is 14 bytes.
If sending a VLAN tagged frame (when env variable vlan is set) this
should be VLAN_ETHER_HDR_SIZE=18 which is what NetSetEther returns.

Signed-off-by: Norbert van Bolhuis <nvbolhuis@aimvalley.nl>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>