This commit adds a dependency mechanism to image creation, so that we can
split the images creation execution in groups, that can be executed in
parallel, having the dependencies satisfied in the same time. The old
code didn't need this since everything was serialized.
Technically, it adds a dependency graph topological sort class that the
main Image class can use to sort out the dependencies.
Images that have dependencies have to declare them using the NEW
IMAGE_TYPEDEP variable, like in the example below:
