Introduce and use a common/remote/Makefile.features.
The boards were already using the common `Makefile.include` and
`Makefile.dep`.
This makes 'include $(RIOTCPU)/cc2538/Makefile.features' be done in
the same module where 'CPU' is defined.
Preparation for moving 'CPU' definition to Makefile.features.
ec312492ee