boards: include potential features from CPU for atmega boards

Features must be provided by the board if they're actually
available on board. Other features might be provided by the
CPU.

Some grouping is also removed as it is not necessary.

boards/common/arduino-atmega/Makefile.features

@@ -11,6 +11,3 @@ ifeq (,$(filter jiminy-mega256rfr2,$(BOARD)))
   FEATURES_PROVIDED += arduino
   FEATURES_PROVIDED += periph_pwm
 endif
-
-# The board MPU family (used for grouping by the CI system)
-FEATURES_MCU_GROUP = avr8

boards/jiminy-mega256rfr2/Makefile.features

+# This board is based on an atmega CPU, thus import the features from it
 include $(RIOTBOARD)/common/arduino-atmega/Makefile.features
 
 # Put defined MCU peripherals here (in alphabetical order)
 # Peripherals are defined in common/arduino-atmega/Makefile.features
 # Add only additional Peripherals
 
-# The board MPU family (used for grouping by the CI system)
-FEATURES_MCU_GROUP = avr6
-
-include $(RIOTCPU)/atmega256rfr2/Makefile.features
+-include $(RIOTCPU)/atmega256rfr2/Makefile.features

boards/mega-xplained/Makefile.features

@@ -8,7 +8,4 @@ FEATURES_PROVIDED += periph_uart
 
 # Various other features (if any)
 
-# The board MPU family (used for grouping by the CI system)
-FEATURES_MCU_GROUP = avr8
-
-include $(RIOTCPU)/atmega1284p/Makefile.features
+-include $(RIOTCPU)/atmega1284p/Makefile.features

boards/waspmote-pro/Makefile.features

@@ -8,7 +8,4 @@ FEATURES_PROVIDED += periph_uart
 
 # Various other features (if any)
 
-# The board MPU family (used for grouping by the CI system)
-FEATURES_MCU_GROUP = avr8
-
-include $(RIOTCPU)/atmega1281/Makefile.features
+-include $(RIOTCPU)/atmega1281/Makefile.features