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.
Some DEFAULT_MODULEs in tree have dependencies of their own. This is
usually done for aliases of init modules, and do not have dependencies
themselves.
This adds the final stage to dependency resolutions where DEFAULT_MODULEs
dependencies MAY be included. These included modules MUST NOT have
dependencies themselves.
This allows for modules to disable DEFAULT_MODULEs during dependencies
resolution independent of the inclusion order.
auto_init and periph_init modules are moved to this outer-loop, allowing
therefore for modules to disable them during the dependency resolution
