esp-netif: Support non-lwip mode, add test

It is required to define a mandatory dependency on lwip, so we
introduced esp_netif_stack component and made it require lwip, instead
of directly depending on lwip.
This enables building w-out lwip and support other TCP/IP stacks.

components/esp_netif_stack/CMakeLists.txt

@@ -0,0 +1 @@
+idf_component_register(REQUIRES lwip)

components/esp_netif_stack/README.md

@@ -0,0 +1,20 @@
+# ESP-NETIF stack component
+
+This component is a direct dependency of ESP-NETIF and it defines a required dependency on lwIP.
+
+## Purpose
+
+Purpose of this component is to pull specific TCP/IP stack (lwIP) into the list of dependencies when using component ESP-NETIF.
+By means of `esp_netif_stack` component, we can define these two
+dependency scenarios:
+
+1) Defines a required dependency on lwIP via this component (default)
+2) In case a non-lwip build is required.
+
+## Configure ESP-NETIF for building without lwIP
+
+In order to use ESP-NETIF without lwIP (e.g. when using custom TCP/IP stack), follow these steps
+
+* unselect `CONFIG_ESP_NETIF_TCPIP_LWIP` in `esp_netif` component configuration
+* add a component `esp_netif_stack` to your private component paths
+* register an empty component `idf_component_register()` in the component's CMakeLists.txt