Add support for bare-metal AVR

[kevinpt]

I've added some changes to support using TaskScheduler on bare-metal AVR without the Arduino support libraries. Additional minor work is needed to do the same for ESP8266 and ESP32.

[arkhipenko]

Thanks. Will look into this after the holidays.

[arkhipenko]

Hi,
I am a little concerned about this part:

#  if !(defined(ARDUINO_ARCH_AVR) || defined(ARDUINO_ARCH_ESP8266) || defined(ARDUINO_ARCH_ESP32))
#    error Missing ARDUINO_ARCH_* definition.
#  endif

Will it nor break other platforms, like teensy or other non-AVR ones which supply their own Arduino.h?
I have never implemented this for bare-bones, so help is appreciated.

[arkhipenko]

I am thinking further: wouldn't a simpler and safer (from the compatibility perspective) way be to include a small local Arduino.h include with your changes for the bare-metal implementations? This way there is no need to change the library at all.
(I am just trying to avoid platform-specific limitations as much as I can).
What do you think?

[arkhipenko]

Upon further thinking I went with a explicit brute force approach:

#ifdef _TASK_BARE_METAL
	extern unsigned long micros(void);
	extern unsigned long millis(void);
	extern void yield(void);
	extern void delay(unsigned long);
#endif

All sketches seem to compile fine.
Will push this into the testing branch. Could you please test once it is there?

Thanks

[arkhipenko]

Updated in the testing branch. Please test.

[arkhipenko]

As a matter of fact I don't think TaskScheduler uses yield() or delay(). I can just go with those:

#ifdef _TASK_BARE_METAL
	extern unsigned long micros(void);
	extern unsigned long millis(void);
#endif

[arkhipenko]

Will give you a little more time to test if you wish, otherwise will close this request.

[kevinpt]

I believe the plain ARDUINO define is also used if you want a platform independent check for the Arduino environment.

[arkhipenko]

Pushed v2.6.0 into the "testing" branch with bare metal fwd definitions. Could you please test?
https://github.com/arkhipenko/TaskScheduler/tree/testing