Ethernut 5 board support

Add support for the Ethernut 5 open hardware design, based
on Atmel's AT91SAM9XE512 SoC.

V4
 - Fix several coding style issues.
 - Move machine type to config file.
 - Remove use of CONFIG_ATMEL_LEGACY.

Signed-off-by: Tim Schendekehl <tim.schendekehl@egnite.de>