- add some xtimer specific defines some tests rely on
- provide the following missing functions:
- xtimer_ticks()
- xtimer_set_timeout_flag()
- xtimer_set_timeout_flag64()
- xtimer_spin()
Add a function that sleeps for ms instead of µs.
This will make the conversion to ztimer easier for users as now there is
a direct mapping to ZTIMER_MSEC.