Some switch configurations will refuse to enable our port unless we
can speak LACP to inform the switch that we are alive.  Add a very
simple passive LACP implementation that is sufficient to convince at
least Linux's bonding driver (when tested using qemu attached to a tap
device enslaved to a bond device configured as "mode=802.3ad").

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