Default address selection for IPv6 should follow RFC 6724

[mike-scott]

Is your enhancement proposal related to a problem? Please describe.
Currently, Zephyr uses a simple diff function to determine the IPv6 source address for outbound data. In many cases this isn't correct (read as: any use case that has more than 1 non-link local unicast address). As a result, OpenThread routing via a border router for internet communication is broken.

Describe the solution you'd like
RFC 6724 (https://tools.ietf.org/html/rfc6724) describes the correct process for selecting the default source address.

Describe alternatives you've considered
I brought up a solution for OpenThread source address, and the RFC was mentioned as the correct solution.

Additional context
none

[mike-scott]

This was brought up as a discussion in #12343

[mike-scott]

NOTE that OpenThread adds a couple of rules:

• Don't use addresses returning true for IsAnycastRoutingLocator()
• Add rule: Prefer EID

[jukkar]

Going through old issues. This is a good candidate for closing as the address selection seems to work ok atm. Can be re-opened if really needed.