[TF-M] Avoid osThreadJoin()

Hello,

The TFM test applications are using CMSIS-RTOS2 API. And this is good. But could you delete os_wrapper_join_thread()and avoid using of osThreadJoin(). If I understand well, its support by RTOS tasks is optional (via osThreadJoinable flag).

The problem is that the osThreadJoin() functionality is not supported by FreeRTOS (https://arm-software.github.io/CMSIS-FreeRTOS/General/html/tech_data.html). It is critical obstacle.

Could you tell what are you going to do with this requirement? To understand, if to continue with a FreeRTOS port.

Thank you Andrej Butok