Since _esp_wifi_recv_cb is not executed in interrupt context but in the context of the `ets` thread, it is not necessary to pass the`NETDEV_EVENT_ISR` event first. Instead, the receive function can be called directly which result in much faster handling, a less frame lost rate and more robustness.

Since _esp_wifi_recv_cb is not executed in interrupt context but in the context of the `ets` thread, the receive function can be called directly. There is no need for a mutex anymore to synchronize the access to the receive buffer between _esp_wifi_recv_cb and _recv function.

When the size of a received frame is checked, always the total length should be used instead of the length of the first lwIP pbuf in the pbuf chain. Otherwise, the check that the length does not exceed ETHERNET_MAX_LEN will always be true since the maximum size of one lwIP pbuf in a pbuf chain is 512 bytes.

Receiption of a frame in _esp_wifi_recv_cb while sending has no effect and should be possible to increases the performance.

Although only the station interface is needed, the WiFi interface has to be used in SoftAP + Station mode. Otherwise the send function blocks sporadically. Since the SoftAP interface is not used, it is configured with a hidden SSID and a long beacon interval. Connections from other stations are not allowed.

A timeout was introduced if sending a frame takes to long time. In that case the WiFi interface disconnects and reconnects automatically to recover.

A reconnect timer is used to try to reconnect every 20 secons if WiFi interface is in disconnected or connecting state.

Due to performance reasons _esp_wifi_recv_cb and _send have to be executed in IRAM.

To avoid conflicts on receiption of a frame while sending, _esp_wifi_recv_cb should not executed while a frame is in transmission

Overriding ethernet_input function _esp_wifi_recv_cb is only used when the WiFi interface is in connected state.

wifi_station_connect and wifi_stattion_disconnect never produce errors. Thus, error handling can be removed.

Different connection states defined to be able to realize a better connect/reconnect handling

Since complete MAC frames are handled, ETHERNET_MAX_LEN has to be used instead of ETHERNET_DATA_LEN for receive buffer size and length check.

Fixes sporadic blocking of the wifi thread in esp_wifi_recv_cb function under heavy network load conditions when frames are coming in faster than they can be processed. Since esp_wifi_recv_cb function is not executed in interrupt context, the msg_send function used for ISR event can block when the message queue is full. With this change esp_wifi can be flooded with icmpv6 packets of maximum size without any problems over hours.