Bluetooth: Introduce skeleton for settings-based storage

Introduce a basic skeleton for peristent storage based on the settings
subsystem. Also enable support for this to the peripheral sample
application, so the new code gets exersized by CI. For now, the
implementation provides the same level support as the bt_storage API
ever did, i.e. for the identity address and the IRK.

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