drivers: wifi: Add ESP8266 and ESP32 wifi modem driver

This adds support for the Espressif ESP8266 and ESP32 devices to be used
as peripherals on a UART.

There are two main AT command versions that can be selected, 1.7 and
2.0. Since they behave a bit different it is important to select the
one that matches the used in the firmware on your device.

When downloading large amounts of data it is highly recommended to
enable CONFIG_ESP_PASSIVE_TCP and flow control on the UART so that
data is not lost due to UART speed or receive buffer size.

Currently unsupported:
- Changing UDP endpoint with a sendto()
- Bind to a specific local port
- Server socket operations, ie listen() and accept()

Official AT firmware for ESP8266 and ESP32 can be found at:
https://github.com/espressif/esp-at



Signed-off-by: Tobias Svehagen <tobias.svehagen@gmail.com>