Add ESP32 def instance & configuration for GR_LYCHEE

[MarceloSalazar]

Find here an enhancement to add a default network interface for the ESP32 wifi driver and the GR_LYCHEE platform.

The simplifies the setup, improves UX for users, and makes it possible to work with automated tests - see here.

Example of usage (main.cpp):

#include "mbed.h"

int main(void) {
    NetworkInterface *net = NetworkInterface::get_default_instance();
    nsapi_error_t net_status = net->connect();
}

@d-kato @toyowata
Please let me know if you have any comments/questions. Thanks!