When IPv6 is prefered it will choose a link local address on a network
that lacks IPv6 over a working IPv4 network, breaking networking.
Change this behaviour to prefer a link local address only as a last
resort.

This also changes LWIP::get_ipv6_addr to not return link local
addresses. Use get_ipv6_link_local_addr instead for this.

Add license identifier to files which Arm owns the copyright to,
and contain either BSD-3 or Apache-2.0 licenses. This is to address
license errors raised by scancode analysis.

- Public headers moved into mbed-os/connectivity/lwipstack/include/lwipstack folder
- Source moved into mbed-os/connectivity/lwipstack/source folder
- Internal lwip and lwip-sys libraries moved to source folder