Change the default IPv6 prefix from aaaa::/64 to fd00::/64

[simonduq]

This PR brings #712 back to life, now with fd00 rather than fc00. As mentioned here I (and some others) believe fd00 to be a better alternative than the Sixxs prefix.

[joakimeriksson]

This should just go in! It is much better than the aaaa:: prefix as we always have been using! 👍

[alignan]

👍 after resolving conflicts

[simonduq]

rebased, conflict resolved, thanks!

[joakimeriksson]

Why is this not just getting in? 👍 from me again! We need to get rid of aaaa:: and have something that is a valid address for the default (local-global) prefix which fd00:: is!

[simonduq]

Rebased again, conflicts solved. This is going to affect many external examples / tutorials etc, so I'd like to see some extra support before merging.
@laurentderu, any thoughts from a 6LBR point of view?
@adamdunkels @g-oikonomou?

[g-oikonomou]

Certainly better than aaaa::! 👍

[laurentderu]

For 6lbr, I think there will be a surge in support request but no real issues . The network prefixes are either user configurable or auto-configured, and the non regression tests are already using different prefixes, so everything should be fine.

[simonduq]

merging, then!