sample: nvs: add NVS sample Kconfig

- adds the ability to change the NVS sample "Maximum reboot counter",
  "Reboot countdown" and its "Sleep time" parameters via Kconfig,
  without manually modifying the source code.
- reduces the default number of resets to 5 (was 400).
- adds the ability to reduce flash memory wear on real devices
  by reducing the number of write iterations.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>