cpu/atmega32u4: make THREAD_STACKSIZE_IDLE configurable
cpu/atmega32u4: make THREAD_STACKSIZE_IDLE configurable

jasonatran/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,yogo1212/RIOT,kYc0o/RIOT,yogo1212/RIOT,yogo1212/RIOT,miri64/RIOT,basilfx/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT,basilfx/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,basilfx/RIOT,miri64/RIOT,OlegHahm/RIOT,basilfx/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OTAkeys/RIOT,yogo1212/RIOT,yogo1212/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,basilfx/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,yogo1212/RIOT,OTAkeys/RIOT