Activate JLink's Zephyr-aware debugging

[maxgerhardt]

Per https://community.platformio.org/t/is-platformio-nrf52-debugger-zephyr-rtos-aware/26965.

Having this flag there will cause the JLinkGDBSever to load GDBServer/RTOSPlugin_Zephyr.dll (or .so) and discover Zephyr-RTOS threads and communicate that to GDB. I.e, Zephyr thread-aware debugging can be done.

A precondition to this though is that

CONFIG_THREAD_MONITOR=y
CONFIG_DEBUG_THREAD_INFO=y

is activated in the zephyr/prj.conf, otherwise the necessary symbols won't be there for the JLink tool to find (ERROR: Mandatory symbol _kernel_openocd_offsets not found). This should be documented somewhere.

[maxgerhardt]

Note: For debug servers / probes using OpenOCD, a separate modification needs to be made: https://community.platformio.org/t/is-platformio-nrf52-debugger-zephyr-rtos-aware/26965/3?u=maxgerhardt. This is however untested and not touched on in this PR. I don't even know if we can just add this -rtos auto flag externally in the OpenOCD invocation or if .cfg files really need to be touched.

[ivankravets]

Does it make sense to check if framework = zephyr is set in the config?

[maxgerhardt]

Oh actually yes. Especially if mbed-os etc. is possible and the JLink software has other plugs for that too. By default it has ChibiOS, embOS, FreeRTOS and NuttX, so not the Keil RTX5 used in mbed-os, but there are extensions for that..