RTOS Support Preview Release

This is a preview release of RTOS support. Currently has only bee tested with FreeRTOS, but should in theory work with the other other RTOS's supported by the chosen debug server.

If you are using FreeRTOS with OpenOCD you will need to need to include the appropriate RTOS helper file from OpenOCD (no modifications are needed for J-Link) - see https://github.com/arduino/OpenOCD/blob/master/contrib/rtos-helpers/FreeRTOS-openocd.c

It should be noted that attempting to step through code before the RTOS has been initialized tends to result in unpredictable behaviour. For example:

• OpenOCD will tend to behave erratically if you try to do anything before vTaskStartScheduler() is called in FreeRTOS (I expect there will be some similar point for other RTOS's)
• The J-Link GDB server will hard crash if you attempt to step through your startup code (initialization of bss, static data, etc. - likely because of random data in device memory); although you can usually step through your FreeRTOS task initialization.

If you need to debug your startup code you should disable the RTOS support, only using RTOS support once tasks are actually running within the RTOS.