Zephyr release 2.1 changed the default settings back end from FCB to
NVS in all Bluetooth samples for performance and reliability reasons.
Update the settings documentation to provide a clue to developers
trying to decide which to use.
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Name all subsystem reference consistently with an '_api' postfix and
clean up naming and folder structure in some cases.
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>