Using a mutex for critical section handling with portENTER_CRITICAL and portEXIT_CRITICAL does not work for RIOT, as this function can also be called in the interrupt context. Therefore, the given mutex is not used. Instead, the basic default FreeRTOS mechanism for critical sections is used by simply disabling interrupts. Since context switches for the ESP32 are also based on interrupts, there is no possibility that another thread will enter the critical section once the interrupts are disabled.
