ztimer_clock are meant to be chained. At the end of the chaine
there is always a timer device object (periph_rtt/rtc/timer).
Since ZTIMER_MSEC and ZTIMER_USEC can be the scaled/shifted with
respect to the base periph_rtt/rtc/timer it makes sense to chain
other ZTIMER_XXX on top of the base rtc/timer/rtt in order to avoid
chained convertions.
Previously, ztimer would happily set an absolute RTT alarm value that exceeds
RTT's maximum value (though not a longer interval), as the `val` was
simply added to `rtt_get_counter()`.
This commit ensures that the target value wraps around RTT_MAX_VALUE.
Fixes#13920.
With #10970 only existing *.c files will be added to SRC when using
the SUBMODULES mechanism, so SUBMODULES_NOFORCE (used to filter out
non existing source files) is now redundant so remove the usage.
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.
