Revamp the image build orchestration

The way images are defined currently is hard to read, tricky to modify and non-obvious. Rather than spreading out complexity all over, better concentrate it in the code, leaving the configuration readable.

After that, declare a dependency of SDK images on armhf development, to allow the future inclusion of the ospack into the SDK images as a part of T5423. This is done by adding a requires parameter, which will create several sub-stages to build the Arm image bits while building the SDK image.

This is currently being tested at: https://jenkins.apertis.org/job/playground/job/andrewsh/job/debos-images-build/lastBuild/flowGraphTable/