This is a temporary fix for Issue #17060. It allows to disable
auto inclusion of `ztimer_periph_rtt` in cases where another
module or application requires direct access.
Limitations:
- as ifeq are involved order of inclusion matters, therefore
these modules should be included early in the build at application
level and not in modules `Makefile.dep`
- this does not disallow direct inclusions of `ztimer_periph_rtt`,
since this only disables auto inclusion of these modules
This is a temporary solution since this is already possible with
Kconfig, but not in make.
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