Obviously I'm a bit biased, but if your embedded platform supports
sockets or serial I second the idea of building off of a bridge (e.g.
rosbridge, http://www.ros.org/wiki/rosbridge or rosserial,
http://www.ros.org/wiki/rosserial ) instead of trying to get a full
ROS-compatible message stack on the embedded platform itself.

Many have reported success using rosserial with arduino and I've had
good results with using rosbridge from both Aurduinos and PICAxes
(after building a serial->socket client). If you're using a Ethernet
shield, there won't even be the need for the serial adapter.

_Trevor