Segger Sysview support

[hathach]

to monitor task, memory and thread resource for high level debug level3+

https://www.segger.com/jlink-rtt.html

[hathach]

Realtrace using sysview is added at Debug Level 3, "Jlink required" text make sure user won't enable it by curiosity (there is no ham except for more SRAM&FLASH).

Click on connect, jlink should be able to detect the mcu + the speed

sysview buffer is likely to overflow, due to the constant switching caused by yield in loop() (I guess). May need to increase SYSVIEW buffer (currently 2*1024 bytes). Nevertheless the captured events look good

All the tasks and its active timeslice is visualized