ENYO-3638: Config.xml: Remove default value on the height width images attributes	

This is a fix for [ENYO-3638] (https://enyojs.atlassian.net/browse/ENYO-3638).
(Ready to be merged).

This makes the config.xml generated by Ares less verbose
- removes default width/height on icon/splash when not defined by operator
- removes platform specific icons/splashs when not defined specifically to a platform
- removes obsolete role attribute for icon/splash

Enyo-DCO-1.1-Signed-off-by: Francois Connetable <francois.connetable@hp.com>