add rtos systick entrance

[Makeblock-official]

add rtos systick entrance for freertos
the free rtos porting placed at https://github.com/Makeblock-official/FreeRTOS-Arduino

[cmaglie]

@Makeblock-official
We did a similar patch for the Arduino Due, basically it adds hooks for

• sysTick
• svc
• pendSV

https://github.com/arduino/Arduino/blob/master/hardware/arduino/sam/cores/arduino/hooks.c
https://github.com/arduino/Arduino/blob/master/hardware/arduino/sam/cores/arduino/cortex_handlers.c#L46

Doing the same for the Zero may solve your problem?

[cmaglie]

Superseded by #13
@Makeblock-official please let me know if that works for you.