[ethernet] Make LACP support configurable at build time

Add a build configuration option NET_PROTO_LACP to control whether or
not LACP support is included for Ethernet devices.

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

src/config/config_ethernet.c

 #ifdef NET_PROTO_STP
 REQUIRE_OBJECT ( stp );
 #endif
+#ifdef NET_PROTO_LACP
+REQUIRE_OBJECT ( eth_slow );
+#endif

src/config/general.h

 #undef	NET_PROTO_IPV6		/* IPv6 protocol */
 #undef	NET_PROTO_FCOE		/* Fibre Channel over Ethernet protocol */
 #define	NET_PROTO_STP		/* Spanning Tree protocol */
+#define	NET_PROTO_LACP		/* Link Aggregation control protocol */
 
 /*
  * PXE support

src/net/ethernet.c

 
 /* Drag in Ethernet configuration */
 REQUIRE_OBJECT ( config_ethernet );
-
-/* Drag in Ethernet slow protocols */
-REQUIRE_OBJECT ( eth_slow );