Jenkinsfile: refactoring

Changes:

• all logical parts are moved to appropriate functions;
• stages are used only in main buildImages() and high-level functions to avoid nested stages usage to have a clean and comprehensive view of the job in Jenkins;
• use separate OSTree repositories to avoid confusion for parallel ostree tasks;
• use individual build statuses per image/tarball. With try/catch companion it allows to continue the build of deriviatives if some image fail to build;
• all artifacts deployed at stage 'upload' after the build;
• tests started only for successfully created images;
• allow to avoid the build image artifacts -- allow to create sysroot without the development image;
• for Armhf only minimal images and sysroot are created.

Signed-off-by: Denis Pynkin denis.pynkin@collabora.com