LWIP::get_ip_addr: Prefer IPv4 over IPv6 link local addresses

[Jookia]

Summary of changes

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.

Impact of changes

This fixes preferring IPv6 on IPv4-only networks.

Migration actions required

Documentation

This might break the API for applications that rely on link local addresses coming from get_ipv6_addr

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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**This is a duplicate of ARMmbed#15423 **

[multiplemonomials]

Looks good, thanks!