autoinstall: add ms-be3004's second NIC to dhcpd

ms-be3004 has a strange issue where the primary NIC cannot PXE, with no
packets arriving on brewster at all, despite being on the right LAN and
everything. This may be an esams core router issue or, more likely, some
bootrom bug. We should probably try a new firmware version for the NIC
bootrom, but in the meantime we've disabled the PXE flag from the
primary NIC and enabled it on the secondary one, which is also cabled.

This has been confirmed to work, with the system booting off the
secondary NIC but then the installer DHCPing and installing properly
from the primary normally.

This is a very strange way to operate, so add the MAC address to
dhcp.conf and document it in comments to avoid further confusion in the
future.

Change-Id: I57f95aadcb7a3af0c93c7418628fc0716b87cad8