[ethernet] Expose eth_broadcast as a global constant

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

src/include/ipxe/ethernet.h

@@ -79,6 +79,7 @@ static inline int is_valid_ether_addr ( const void *addr ) {
 		 ( ! is_zero_ether_addr ( addr ) ) );
 }
 
+extern uint8_t eth_broadcast[];
 extern int eth_push ( struct net_device *netdev, struct io_buffer *iobuf,
 		      const void *ll_dest, const void *ll_source,
 		      uint16_t net_proto );