cpu/stm32f1: improve rtc irq
cpu/stm32f1: improve rtc irq

RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,basilfx/RIOT,mtausig/RIOT,ant9000/RIOT,toonst/RIOT,authmillenon/RIOT,smlng/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,josephnoir/RIOT,miri64/RIOT,toonst/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,basilfx/RIOT,yogo1212/RIOT,kYc0o/RIOT,OlegHahm/RIOT,x3ro/RIOT,smlng/RIOT,yogo1212/RIOT,ant9000/RIOT,kaspar030/RIOT,basilfx/RIOT,basilfx/RIOT,mtausig/RIOT,josephnoir/RIOT,toonst/RIOT,mtausig/RIOT,OTAkeys/RIOT,x3ro/RIOT,josephnoir/RIOT,kaspar030/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kaspar030/RIOT,x3ro/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,smlng/RIOT,x3ro/RIOT,authmillenon/RIOT,x3ro/RIOT,jasonatran/RIOT,OTAkeys/RIOT,smlng/RIOT,ant9000/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,toonst/RIOT,yogo1212/RIOT,basilfx/RIOT,jasonatran/RIOT,josephnoir/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,smlng/RIOT,kYc0o/RIOT,mtausig/RIOT,kYc0o/RIOT,toonst/RIOT,jasonatran/RIOT,mtausig/RIOT,OlegHahm/RIOT