Protocols like mdns requires IPv6 link local address to be advertised in its
records (AAAA record). LWIP::Interface::bringup() API is creating IPv6 link
local address;But as of now there is no API exposed by mbed-os to get the
IPv6 link local address.
This new API is required to deliver mDNS library support on mbed-os for Cypress
platforms. Unit tested it by invoking get_ipv6_link_local_address with a simple
application.