[dhcp] Use Ethernet-compatible chaddr, if possible

[dhcp] Use Ethernet-compatible chaddr, if possible

For IPoIB, we currently use the hardware address (i.e. the eight-byte
GUID) as the DHCP chaddr.  This works, but some PXE servers (notably
Altiris RDP) refuse to respond if the chaddr field is anything other
than six bytes in length.

We already have the notion of an Ethernet-compatible link-layer
address, which is used in the iBFT (the design of which similarly
fails to account for non-Ethernet link layers).  Use this as the first
preferred alternative to the actual link-layer address when
constructing the DHCP chaddr field.

Signed-off-by: Michael Brown <mcb30@ipxe.org>