* esp-netif to optionally depend on esp-eth (only for l2tap config)
* esp_eth.h now includes the original ethernet header and the
ethernet-netif glue layer
* Updated examples and test to explicitely use esp-eth dependency if
needed
simple_sniffer was moved from /wifi folder to /network folder
network_tests was moved from /system to /network folder
README's were updated accordingly to reflect new structure