Fix dependency tree so that lwip doesn't depend on any specific network
interface component.
Network interface drivers shall depend on esp_netif.
esp_netif shall depend on lwip (but not on any specific interface
driver) -- it optionally depends on vfs and esp_eth (need ethernet
header for L2/bridge mode)
Also added guide for creating a custom network capable I/O driver
And added notes about default wifi interfaces and implications of using
default short-hand creation functions.