With this commit xtimer will always be implemented with ztimer.
The default compat layer will be ztimer_xtimer_compat, so xtimer
calls will be inlined to ZTIMER_USEC, or ZTIMER64_USEC when the
64bit xtimer API is required.
This can be reverted by adding 'xtimer_no_ztimer_default' module
to a build.
This adds a xtimer_no_ztimer_default that is currently always
selected in Makefile, but that can be switched off in Kconfig.
Removing its inclusion will allow switching the default xtimer
backend to ztimer, while allowing for an easy way back.
