router: move pref64 calculations to the config stage

router: move pref64 calculations to the config stage

If pref64 is misconfigured, it'll print a LOG_WARNING message every time a RA
is generated, which seems a bit excessive.

When fixing that, I noticed that all work is done for every RA, so I ended up
moving the gist over to the config stage where it is done once.

Signed-off-by: David Härdeman <david@hardeman.nu>