[LoRaWAN] Change session activation

[StevenCellist]

Due to a slight bug introduced in one of my recent patches, I decided after conferring with @HeadBoffin to shift around some of the logic around session restoring / activation.

Restoring the Nonces and Session buffer now immediately restores all the contents of the buffer, instead of waiting for this to happen in begin(). Should make it easier to read and maintain. Now if begin() is called with the force argument set to false (default), it will check if a restored session is available for the current activitation information (keys, plan and the like) and return SESSION_RESTORED if this is the case - if there was none or not a valid session, it will immediately do a join and return (if succeeded) NEW_SESSION. Hence there is no need to set force to true anymore, simplifying logic and being more intuitive. It is still available for rejoining while a session is active, however.

For the examples in this repository there is not much difference (just begin()'s return code and dropping the argument to begin()), but the persistence examples should see nice improvements.

Also, a slight bug has been fixed regarding Nonces not increasing on a failed join (buffer checksum was not recalculated and could result in restoring rejection).

Pending confirmation by @HeadBoffin (already tested to run fine, but slight misuse-testing also welcome), and ready for review.

[jgromes]

Thanks again @StevenCellist - looks good from my end! Admittedly I can't really check the complete join/restore logic, but from the POV of code clarity, style, memory handling etc. all seems to be in order :)

[StevenCellist]

The latest commits split the begin() out into begin() and activate().
The reasoning is as follows: to restore the nonces and session, the code should already know what it should expect w.r.t. keys, mode, frequency plan and class. If you first restore and then call begin() with a different key, it must throw away whatever it restored and it is hard to make it clear from return codes that something like that happened.
It is much easier if we first call begin() so the code knows what to expect, then restore the buffers, and finally call activate() - this either finalizes the restored buffers, or does a fresh join.

• begin() is now a void-function as it just takes the keys, generates their checksums and sets mode and class (latter always A)
• setBufferNonces() returns ERR_NONE or NONCES_DISCARDED (or CHECKSUM_MISMATCH)
• setBufferSession() returns ERR_NONE or SESSION_DISCARDED (or CHECKSUM_MISMATCH)
• activate() returns SESSION_RESTORED or NEW_SESSION (or NO_DOWNLINK or any hardware-related error-code)

Additionally, an optional event is added to activateOTAA() that returns the DevNonce and JoinNonce.

Checked to run fine on deepsleeping ESP32s for both OTAA and ABP on EU868, but waiting to get verified by an independent tester (@HeadBoffin).

[StevenCellist]

Additionally, the user now has access to clearing a session, after which calling activate() results in starting a new session. Thus, the force argument in the activate() functions is now dropped to make the flow more explicit if one wishes to start a new session while one is active.
After calling clearSession(), the function isActivated() evaluates to false which is an easy indication to call activate().

[HeadBoffin]

Abbrviating th function nams and th rturn cods dosn't mak things straightforward. Fundamntally works but thr is somthing in th saving of th join on boot == 1 that causs a join on boot == 2. Thraftr it sms to procd normally.

[StevenCellist]

@HeadBoffin I think your 'e'-key needs replacement ;)

[jgromes]

Since begin methods are now void, and new activate methods were added, we need to update the examples to reflect that.

[StevenCellist]

Yup.. Nick or I will follow up with that!

[HeadBoffin]

A product of the testing!

[StevenCellist]

Examples rework in progress, via separate PR.