Bluetooth: Mesh: Separate adv tag/set for friend

The purpose of this commit is to facilitate future improvements to the
LPN and friendship feature. By being able to identify friendship related
messages on the advertising layer, it will be possible to treat these in
a specific manner so that message exchange between LPN and friend
devices can be conducted in a more power efficient manner.

This commit adds the following:
   - A separate tag for friendship related messages
   - A optional separate advertising set for friend related messages

Signed-off-by: Anders Storrø <anders.storro@nordicsemi.no>