Add support for FreeRTOS

[Yveaux]

Compiles for FreeRTOS when FREERTOS is defined.

Note that _WIN32 and FREERTOS can coexist!

[SergiusTheBest]

Thanks for the PR! What kind of FREERTOS are you using? I'm worried about __FREERTOS__ macro as it seems not a standard macro defined by FreeRTOS itself.

[Yveaux]

Thanks for the PR! What kind of FREERTOS are you using? I'm worried about __FREERTOS__ macro as it seems not a standard macro defined by FreeRTOS itself.

I searched for a standardized way to detect FreeRTOS usage during compilation but there doesn't seem to be one. https://www.freertos.org/FreeRTOS_Support_Forum_Archive/February_2012/freertos_Lib_with_conditional_compiling..._5057945.html suggests to use INC_FREERTOS_H but that's not supported on all compilers and seems tricky.

Therefore I picked __FREERTOS__ but any other will be just as good; I'm open to suggestions.

As I'm working on embedded code that cross compiles to FreeRTOS under windows a combination of _WIN32 and __FREERTOS__ is also supported.

[SergiusTheBest]

I've added a FreeRTOS sample to be automatically built. I didn't find anything better then keep __FREERTOS__ (just added a comment about it).

[Yveaux]

Thanks for the rerwork & merge! 👍