Feature/freefall app

[mcserved]

Summary:

A basic implementation of the Freefall App. The node connects over OTAA and wait for freefall events. If a timeout was reached, then the events are accumulated. The payload is one byte containing the amount of freefalls (-1) that occurred since the previous transmission. Closes #113.

Changes:

• A new folder in the app dir (with the basic_lorawan as base code)
• The changes required to setup the freefall app (mainly in lora_app.c, freefall.c, app_conf.h)
• Initialisation (and detinitialisation) functions for the accelerometer pin in GNSE_bsp_gpio.c
• Changed Cmakelists.txt to add freefall_lorawan as app

Notes for Reviewers:

The default parameters are based on this: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjrl_ftvevuAhVQzYUKHVFlBDUQFjAAegQIBRAC&url=https%3A%2F%2Fwww.st.com%2Fresource%2Fen%2Fdesign_tip%2Fdm00500824-setting-up-freefall-recognition-with-sts-mems-accelerometers-stmicroelectronics.pdf&usg=AOvVaw2EvAql8RUKdAtUcV6iYKOK This amount seems about right, where free falls are detected but rebounds from bouncing on the surface are not. I tested by dropping it on my bed from arms length (yanking it up just right on my desks also works). The second transmissions usually needs around 3 min for the next transmission, you can try dropping it a few times to see if it counts the amount you dropped it.

Release Notes: (optional)

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