lorawan: adding settings NVM for LoRaWAN

Adding a reference implementation of the Non-Volatile Memory module
needed to join any LoRaWAN network.

This NVM is based on the SETTINGS subsys to store all the required
key to join and communicate on a LoRaWAN network.

Without proper NVM, one may experience errors when using OTAA
to join the network, as the device may violate anti-replay
protection (depending on the version of LoRaWAN).

Signed-off-by: Giuliano Franchetto <giuliano.franchetto@intellinium.com>