boards/atmega328p: Fix duplicated includes
boards/atmega328p: Fix duplicated includes

avrdude.mk and serial.mk was included twice. As a result of the former, avrdude
wasted one flash cycle and some time by writing the same firmware twice.

ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,basilfx/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,OTAkeys/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,basilfx/RIOT,basilfx/RIOT,OTAkeys/RIOT,basilfx/RIOT,basilfx/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kaspar030/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,authmillenon/RIOT