Makefile.features: include CPU/Makefile.features when CPU is defined

Prepare for when boards define `CPU` in `BOARD/Makefile.features`.

Include '$(RIOTCPU)/$(CPU)/Makefile.features' directly when it is defined.
This will allow removing the file inclusion from the `BOARD/Makefile.features`.
The board must then not include it directly.

Transitional change to allow migrating part by parts.

Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.

Makefile.features

@@ -10,3 +10,8 @@
 #   reflect the state of all boards for the moment.
 
 include $(RIOTBOARD)/$(BOARD)/Makefile.features
+
+# Transitional conditional include until all boards define 'CPU'
+ifneq (,$(CPU))
+  include $(RIOTCPU)/$(CPU)/Makefile.features
+endif

Makefile.include

@@ -261,7 +261,7 @@ include $(RIOTCPU)/$(CPU)/Makefile.include
 # The check is only done after 'include $(RIOTBOARD)/$(BOARD)/Makefile.include'
 # because we need to have the 'CPU' variable defined
 ifeq (,$(filter $(RIOTCPU)/$(CPU)/Makefile.features,$(MAKEFILE_LIST)))
-  $(error $$(RIOTCPU)/$$(CPU)/Makefile.features must have been included by the board / board common Makefile.features)
+  $(error $$(RIOTCPU)/$$(CPU)/Makefile.features must have been included by the board / board common Makefile.features or Makefile.features)
 endif
 
 # Assume GCC/GNU as supported toolchain if CPU's Makefile.include doesn't