First preview version

[matthijskooijman]

This is a slightly more complete and cleaned up version, which should be suitable to privately share with selected customers. There is still a lot of things unfinished, including some licensing details, so this is not ready to be shared publicly yet.

Also, this PR needs a private version of the STM32Duino core as well, which includes updated CMSIS and HAL drivers, and also some changes by me. See https://github.com/stm32duino/Arduino_Core_STM32_WL/pull/2

A single example is included that joins the network (OTAA or ABP supported by changing a single line) and then sends a data packet every 60 seconds.

The identifiers and keys used to join the network can be directly put in the sketch, in the joinOTAA() or joinABP() method calls.

Some notable limitations include:

• Arduino API incomplete (sending does not use Stream yet, no reception, limited configuration)
• EU868 region currently hardcoded
• On OTAA, only on join attempt (one join request message) is made. If that fails, the example sketch loops indefinitely.
• Blocking vs nonblocking and poll() API to reviewed.
• ADR is not supported yet.
• Handling of critical sections left to be reviewed
• Duty cycle limits might not be enforced (left to check)
• No session state is stored, so when using OTAA, LoRaWAN 1.0.3-style random nonces are used (instead of 1.0.4-style incremental nonces) and when using ABP, the framecounter resets on every board reset (which looks like a replay attack to the server, so messages will be dropped unless replay protection is disabled server-side. On TTN this is the "Resets frame counters" checkbox under advanced network settings for the device)
• On ABP, the frequency plan is just the default, which might need to be preconfigured if the network uses different settings. On OTAA, this is handled through the join-accept message.

[matthijskooijman]

I've pushed an update to this PR, the library is largely complete in terms of code now and documentation is also comping along. I have amended some of the old commits a bit, but the main change of this push is this big new commit: 8a2411b03a416a17b35df65b0cae114dbd45467d, see the message for details (plus a few subsequent commits).

[matthijskooijman]

Closing in favor of #3.