Merge pull request #5529 from kYc0o/atmega_common_unify

boards/arduino-mega2560/include/board.h

cpu/atmega2560/Makefile

cpu/atmega2560/Makefile.include

cpu/atmega2560/include/cpu_conf.h

cpu/atmega2560/include/periph_cpu.h

cpu/atmega_common/Makefile

cpu/atmega_common/Makefile.include

cpu/atmega_common/include/atmega_regs_common.h

cpu/atmega_common/include/periph_cpu_common.h

cpu/atmega_common/periph/Makefile

cpu/atmega_common/periph/gpio.c

cpu/atmega_common/periph/spi.c

cpu/atmega_common/periph/timer.c

cpu/atmega_common/periph/uart.c