subsys: Add a new settings subsystem

Adapt the MyNewt non-volatile configuration system to become a settings
system in Zephyr.
The original code was modifed in the following ways:

* Renamed from config to settings
* Use the zephyr FCB, FS API, and base64 subsystems
* lltoa like function was added to sources as it was required but not
  included in Zephyr itself.
* The original code was modified to use Zephyr's slist.h as single
  linked list implementation.
* Reworked code which was using strtok_r, added function
  for decoding a string to a s64_t value.
* Thank to the above the settings subsys doesn't require newlibc anymore.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>