* 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
* Philosophical: "explicit is better than implicit".
* Practical: Allows useful errors if invalid directories given in components as the defaults aren't
always used. Also trims the -I path from a number of components that have no actual include
directory.
* Simplifies knowing which components will be header-only and which won't
