Now that CPU and CPU_MODEL are defined in the board's Makefile.features
it can be used to determine the available features provided by the
specific model.
This removes doing `filter-out periph_hwrng, $(FEATURES_PROVIDED)`
after processing `cpu/$(CPU)/Makefile.features`.
The current solution is a HACK as `CPU_MODEL` is currently not available
at that moment but will be in the near future.
It will allow always including `cpu/$(CPU)/Makefile.features` after
`boards/$(BOARD)/Makefile.features`.
It is a part of moving `CPU/CPU_MODEL` definitions to `Makefile.features`.
