boards/mulle: move CPU/CPU_MODEL definiton to Makefile.features

cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included

The multiple boards handling has been changed to use 'CPU_MODEL ?='.

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

boards/mulle/Makefile.dep

@@ -19,5 +19,3 @@ ifneq (,$(filter saul_default,$(USEMODULE)))
   USEMODULE += saul_gpio
   USEMODULE += saul_adc
 endif
-
-include $(RIOTCPU)/kinetis/Makefile.dep

boards/mulle/Makefile.features

@@ -1,3 +1,13 @@
+CPU = kinetis
+
+### CPU part number (must have a specific linker script for each part)
+# Note that MK60DN256ZVLL10 (version 1.x) and MK60DN256VLL10 (version 2.x, no Z)
+# only differ in some register locations etc, not in the actual memory layout,
+# so it is safe to use the same linker script for both version 1.x and version
+# 2.x silicon.
+# The linker script needs to know the flash and RAM sizes of the device.
+CPU_MODEL ?= mk60dn512vll10
+
 # Put defined MCU peripherals here (in alphabetical order)
 FEATURES_PROVIDED += periph_adc
 FEATURES_PROVIDED += periph_dac
@@ -8,5 +18,3 @@ FEATURES_PROVIDED += periph_rtt
 FEATURES_PROVIDED += periph_spi
 FEATURES_PROVIDED += periph_timer
 FEATURES_PROVIDED += periph_uart
-
-include $(RIOTCPU)/kinetis/Makefile.features

boards/mulle/Makefile.include

@@ -1,24 +1,8 @@
-# define the cpu used by the Mulle board
-export CPU = kinetis
-
 # MULLE_SERIAL is used to select which specific Mulle board we are compiling for.
 ifdef MULLE_SERIAL
   CFLAGS += -DMULLE_SERIAL=$(MULLE_SERIAL)
 endif
 
-### CPU part number (must have a specific linker script for each part)
-# Note that MK60DN256ZVLL10 (version 1.x) and MK60DN256VLL10 (version 2.x, no Z)
-# only differ in some register locations etc, not in the actual memory layout,
-# so it is safe to use the same linker script for both version 1.x and version
-# 2.x silicon.
-# The linker script needs to know the flash and RAM sizes of the device.
-
-ifeq ($(CPU_MODEL),)
-  CPU_MODEL = mk60dn512vll10
-endif
-
-export CPU_MODEL
-
 # Default debug adapter choice is to use the Mulle programmer board
 DEBUG_ADAPTER ?= mulle
 # Host OS name