cpu/esp32: make sure MAXTHREADS is at least 3
cpu/esp32: make sure MAXTHREADS is at least 3

ESP32 has an esp_timer thread in addition to the main and the idle
thread. So applications that work fine with 2 threads on other
platforms will break on ESP32.

ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT