The only messages that should be encrypted using the friendship
credentials are those coming through the Friend Queue on the Friend
node, most request-response pairs between LPN & Friend (exceptions are
Friend Request - Friend Offer, and Friend Clear - Friend Clear
Confirm), as well as Model Publication messages when the Friendship
Credentials Flag has been enabled in the model publication.
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>