cpu: atmega_common: Initial import
cpu: atmega2560: Initial import
board: arduino-mega2560: Initial import
tests: blacklist failing test on arduino-mega2560
sys: inet_pton: conditional include for atmega2560

    include sys/types.h conditionally for atmega2560
    Without this change test/unittests fails to build due to a missing
    declaration of ssize_t
Merge pull request #1420 from N8Fear/initial_import_arduino_mega2560

board: cpu: Initial import of the Arduino Mega2560