cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
The softdevice is only verified to be working on nrf52832-based
boards. This feature prevents the softdevice from being build for
other, similar targets (e.g. nrf52840-based boards).