Remove prefix and default-target, add repo

The concept of prefix was dropped by ostbuild, the default-target will
soon be dropped as well.

We also add the online repository URL too.
p11-kit: Untag

It's been fixed.
ostree, dracut: Regenerate initramfs when these components change

ostbuild now generates initramfs only if certain components change,
this will make the build process faster and saves several megabytes
per build and avoids client from downloading the same initramfs
every update.