boards/common/nrf52: move CPU/CPU_MODEL definiton to Makefile.features

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.

boards/common/nrf52/Makefile.dep

@@ -1,5 +1,3 @@
-include $(RIOTCPU)/nrf52/Makefile.dep
-
 ifneq (,$(filter skald,$(USEMODULE)))
   USEMODULE += nrfble
 endif

boards/common/nrf52/Makefile.features

@@ -1,3 +1,5 @@
+CPU = nrf52
+
 # Put defined MCU peripherals here (in alphabetical order)
 FEATURES_PROVIDED += periph_rtt
 FEATURES_PROVIDED += periph_timer
@@ -8,5 +10,3 @@ endif
 # Various other features (if any)
 FEATURES_PROVIDED += ble_nimble
 FEATURES_PROVIDED += radio_nrfble
-
-include $(RIOTCPU)/nrf52/Makefile.features

boards/common/nrf52/Makefile.include

@@ -1,6 +1,3 @@
-# this module contains shared code for all boards using the nrf52 CPU
-export CPU = nrf52
-
 # include this module into the build
 INCLUDES += -I$(RIOTBOARD)/common/nrf52/include