[WKUP1]PA0 is a WKUP1 key, 10K pull down and 10K pull high.
active high. CPU enter standby mode by pressing 500 ticks.
CPU leaves standby mode by WKUP1 rising edge.

[idle]rt_thread_idle_sethook(cpu_sleep) to setup cpu sleep hook in idle thread
to save power when no threads are running. idle thread is a lowest thread.