feature: add an optional 'image' field to config files.

Some configurations require specific docker images to build (e.g. cca-4worl.yaml needs shrinkwraptool/base-full), and this requirement is exposed on the command line, as the user has to specify the image when invoking shrinkwrap. This seems unnecessary and error prone.

This patch adds support for an 'image' keyword in the configuration file. If no image is specified on the command line, shrinkwrap will use the image from the recipe (or a default one if none is specified). When merging multiple configurations, conflicts (different image names) are detected and reported as we have no way of merging this requirement. At this time, in case of conflict there is no way to resolve it by forcing the image to use from the command line.

Signed-off-by: Arnaud de Grandmaison arnaud.degrandmaison@arm.com