Update Makefile with library support based on
https://github.com/jnmacdnld/arduino-due-makefile/commit/81f8e851652ba3d5a63e03dfe464aedf018f37bf
Make ADIR point at the top-level dir, not .../hardware
Add Ethernet to Makefile
Add Ethernet support.

Supports the W5100 chip with interrupt-driven operation and packet
timestamping. Doesn't have NTP protocol code yet, just runs an echo
server on port 123 for debugging.
Use \r\n for the sake of working with serial monitors that expect it