[AVR32] Move ethernet tag parsing to board-specific code

[AVR32] Move ethernet tag parsing to board-specific code

By moving the ethernet tag parsing to the board-specific code we avoid
the issue of figuring out which device we're supposed to attach the
information to.  The board specific code knows this because it's
where the actual devices are instantiated.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>