Bluetooth: Add full storage support for multiple identities

Update the storage handling to take into account multiple identities.
We can save a bit of code by using the new bt_id_create() API from
within settings.c.

Also make the treatment of addr & irk parameters to bt_id_create()
consistent, in that NULL is acceptable for both of them.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>