[net] fix linux eth define name and constructor name in net_test
[net] use dummy mac address for linux eth simulation
[net] register ethernet protocol handler directly from ethif constructor
[net] rename ETHERIF_RESOURCE_ID_BASE -> NETIF_RESOURCE_ID_BASE
[net] move network device accounting from etherif to netif
[net] introduce netif_ops_t. move mac_addr_get() op there
[net] add netif->free() method