boards/common/arduino-atmega: Refactoring

- including `avrdude.mk` and `serial.mk` is no longer needed
- the avrdude flag -F is dropped
    - This allows flashing a board having an MCU different to the one
      RIOT was compiled for
    - It is better to let the user provide this flag manually if she/he really
      wants to turn safety off, knows what she/he is doing, and accepts the
      risk associated with this
- the avrdude flag -D is dropped
    - This disables issuing an erase command during flashing, however this
      erase cycle is strictly required to be able to program the flash
    - This is only needed in case of the stk500v2 bootloader, which implicitly
      does the erase cycle and fails if explicitly asked to do so
    - Only for the stk500v2 bootloader this flag is now added

boards/common/arduino-atmega/Makefile.include

@@ -1,10 +1,3 @@
 INCLUDES += -I$(RIOTBOARD)/common/arduino-atmega/include
 
-include $(RIOTMAKE)/tools/serial.inc.mk
-
-# Disable auto erase for flash and avoid error if signature doesn't match
-FFLAGS_EXTRA += -F -D
-
-# include avrdude flashing tool
-include $(RIOTMAKE)/tools/avrdude.inc.mk
 include $(RIOTBOARD)/common/atmega/Makefile.include