Arduino Port

[fdgonthier]

With the ESP-ism removed from most of the code, it was reasonable to expect RinaSense to compile as an Arduino platform library. This patch adds the machinery required for this to happen.

The limitation is that this is an ESP32-specific Arduino port, because:

1. Our use of POSIX relies on FreeRTOS+POSIX, which in turns relies on the ESP32 port of FreeRTOS
2. The wifi NetworkInterface is currently exclusive to ESP32

We can still use ESP32-specific code where needed because the Arduino implementation on ESP32 is layered on top of the FreeRTOS implementation.

For a more generic Arduino implementation, the following will need to happen:

1. Integrate a port of FreeRTOS for Arduino.
2. Hope that we can make the pthread implementation in FreeRTOS+POSIX work with said port.

The most likely element to fail is 2), which would force us to write our own thread abstraction in the portability layer.