wire up ZTIMER_SEC to the existing RTC backend, or RTT backend, or periph_timer
backend (in this order of preference).
Update sys/ztimer/auto_init.c
Co-authored-by: Leandro Lanzieri <leandro.lanzieri@haw-hamburg.de>
This commit adds logic to make xtimer use ztimer_usec as backend
(instead of periph_timer). This allows ztimer_usec and xtimer to
coexist. It also allows xtimer to profit from eventually implemented
power mode blocking in ztimer's periph_timer backend.
