New `bin/gencontrol.py` argument:  `--dump-config`

This helps a lot with debugging.

Also fix a couple of objects with broken or missing `__repr__()`.
Allow templating pkg relation config fields

Use case is adding `Provides:` relations to a package, e.g.:

  `Provides: rtai-modules-%(abiname)s-rtai.x86-686-pae`
Add `Provides: rtai-modules-...` to RTAI kernel pkg

This adds compatibility with LinuxCNC project kernel pkgs.
Remove `Provides: linux-image-<featureset>` hard-coded hack

The right way to do this is in the package config.
Add `Provides: linux-image-<xenomai|rtai>` to pkgs

Make other package `Depends:` relations braindead-simple