Sysroot cleanup, plus some other tweaks

• sysroot: Align defaults to the ospack-sysroot recipe
• ospack-sysroot: Drop packages not relevant for cross-building
• sdk: Install apertis-dev-tools/ade again
• ostree-prepare-local-repo: Split ostree repo creation out of Jenkinsfile

This should probably be split in different MRs but it's late so if needed I will do so tomorrow. :)

Here's a test build fwiw.