This function is called when the underlying network device driver needs to notify the lwIP TCP/IP stack of a link (PHY) event. Primarily this would be used to report link up or down events as appropriate, since certain TCP/IP operations may need to occur when a network connection is re-established (e.g. ARP packets, address negotiation, etc.).
ctx parameter should be the pointer
returned by the cyg_net_eth_phy_ctx_acquire call.