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)
This MR adds the OpenThread border router example and supports the
Thread 1.1 border agent feature.
* Adds the OPENTHREAD_BORDER_ROUTER Kconfig option.
* Adds platform UDP and task queue port for the border agent feature.
* Adds `esp_openthread_border_router_*` api.
* Adds the `esp_otbr` example.
