lorawan: unconfirmed messages leave stack in busy state

[JordanYates]

Is your enhancement proposal related to a problem? Please describe.

The current implementation of lorawan_send only waits for the transmission to complete if the LORAWAN_MSG_CONFIRMED flag is set. For unconfirmed transmissions, the application is free to call lorawan_send again before the receive windows for the first transmissions have finished, resulting in lorawan_send returning -EBUSY.

Ideally there would be some application level way to wait until the stack is ready to accept new commands/transmissions.
This can be done without changes to the stack as McpsConfirm is already called when all transmissions complete.

Describe the solution you'd like

The simplest solution I have identified is forlorawan_send to always wait on the mcps_confirm_sem, regardless of the confirmed flag. This would add some application level latency for unconfirmed transitions if the application was already guaranteeing sufficient packet spacing.

Alternative solutions include:

• Adding an additional flag to signify the driver should wait for unconfirmed transmissions to complete.
• Adding a new function that blocks until the stack is ready to accept new commands.

I'm not sure if the second alternative is feasible, as it sounds like waiting on a boolean flag.

Describe alternatives you've considered

The application could either wait for some fixed time period after each unconfirmed transmission, or delay and retry if -EBUSY is returned. Neither solution is very nice, as the LoRaWAN driver already knows when the stack is ready again.