Link borken

[kiskovce]

link for new esp8266 driver is malfunction.

[JanneKiiskila]

Mbed OS code gets re-structured on regular basis and thus links get broken.

• Currently (in master) it's located in; https://github.com/ARMmbed/mbed-os/tree/master/connectivity/drivers/wifi/esp8266-driver
• In 5.15 branch it's located in; https://github.com/ARMmbed/mbed-os/tree/mbed-os-5.15/components/wifi/esp8266-driver

[JanneKiiskila]

Fix in; #125