webos-image: define WEBOS_IMAGE_DEFAULT_FEATURES

* IMAGE_FEATURES shared by all default images

Open-webOS-DCO-1.0-Signed-off-by: Simon Busch <morphis@gravedo.de>
Change-Id: If90f64ae48cc271010228fa79e09211ff3c79ef4
Reviewed-on: http://g2g.palm.com/1094
Reviewed-by: Martin Jansa <martin.jansa@palm.com>
Tested-by: Martin Jansa <martin.jansa@palm.com>
Reviewed-by: Herb Kuta <herb.kuta@palm.com>