Merge pull request #9984 from kaspar030/refactor_atmega_libc

boards/common/atmega/board.c

boards/waspmote-pro/board.c

cpu/atmega_common/Makefile

cpu/atmega_common/Makefile.include

cpu/atmega_common/avr_libc_extra/Makefile

cpu/atmega_common/avr_libc_extra/atmega_stdio.c

cpu/atmega_common/avr_libc_extra/include/errno.h

cpu/atmega_common/avr_libc_extra/include/inttypes.h

cpu/atmega_common/avr_libc_extra/include/sys/time.h

cpu/atmega_common/avr_libc_extra/include/sys/types.h

cpu/atmega_common/avr_libc_extra/include/unistd.h

cpu/atmega_common/avr_libc_extra/include/vendor/sys/stat.h

cpu/atmega_common/avr_libc_extra/include/vendor/time.h

cpu/atmega_common/avr_libc_extra/posix_unistd.c