Makefile.features: add doc on CPU/CPU_MODEL definition in board

Describe the ongoing migration.

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

Makefile.features

@@ -1,4 +1,13 @@
 # Process FEATURES variables
+#
+# The board/board common are responsible for defining the CPU and CPU_MODEL
+# variables in their Makefile.features.
+# This makes them available when setting features based on CPU_MODEL in the cpu
+# Makefile.features and also during dependency resolution.
+
+# Transition:
+#   Moving 'CPU/CPU_MODEL' to Makefile.features is an ongoing work and may not
+#   reflect the state of all boards for the moment.
 
 include $(RIOTBOARD)/$(BOARD)/Makefile.features