lorawan: allow setting of DevNonce for OTAA re-join

Starting with LoRaWAN 1.0.4 the DevNonce sent with the OTAA join must be
monotonically increasing for each new join with the same EUI. The DevNonce
should be stored in non-volatile memory by the application.

This commit uses a simple extension of the lorawan_join_otaa struct to
allow specifying the DevNonce.

Signed-off-by: Martin Jäger <martin@libre.solar>