[lacp] Add simple LACP implementation

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>

src/net/ethernet.c

@@ -191,3 +191,6 @@ struct net_device * alloc_etherdev ( size_t priv_size ) {
 	}
 	return netdev;
 }
+
+/* Drag in Ethernet slow protocols */
+REQUIRE_OBJECT ( eth_slow );