This updates (or adds) a compression context whenever a new prefix
arrives at the border router. This allows 6LoWPAN to compress said
prefix in the network.
Sadly, there is now way to just remove the context when the prefix is
overwritten, so I do not do it. If an administrator chooses to reset the
prefix they can use `6ctx del` which timeouts the prefix appropriately,
but IMHO it doesn't hurt to keep the old contexts.
8cc9c5ebe7