[dhcp] Check for matching chaddr in received DHCP packets

On large networks a DHCP XID collision is possible.  Fix by explicitly
checking the chaddr in received DHCP packets.

Originally-fixed-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Merge branch 'master' into osimages