Merge pull request #37 from hadleyrich/master

Silence warnings
Watchdog timer driver. Maybe even works.
Closes #5
Ringbuffer uses sizes rather than masks in API.
Clarify what the safe/unsafe versions of ringbuffers are doing.
Change usart code to use sizes as well.
Closes #38
Rework SPI API to accept buffers to read/write via
Closes #39
Add an error return for busy (EBUSY, using POSIX number.)
Correct documentation of SPI, you MUST NOT pass the same buffer to tx/rx.
Initial cut at WizNet W5500 driver.
Supports TCP using avr-libc STDIO, UDP using direct read/write.
Currently no support for TCP server sockets on the MCU side.
Fix up name of guard define, was using out-of-tree naming.
Opps, wrong license. LGPL should apply to these files.
Cleanup of w5500 driver.
Add debug guards to all printf() debug output
Remove some commented out code that was used during dev
Fix up doxygen so expected arrays are noted
Remove a few unused variables
Supress warning about w5500_tcp_listen() have no return code since it's empty.
Add SPI TX dummy byte config
Adjust W5500 driver to provide SPI TX dummy byte
Simple NVM driver for various calib values and usersig access.