Makefile: Repatriate the definition of packaging targets.

Prior to this change, the Makefile packaging targets were generated by
a Python script, supposedly to reduce boilerplate.  Make is able to
programmatically define rules, so use its features instead.

The packaging targets 'package-' prefix is dropped, making the targets
names match the distribution directly, such as 'ubuntu_20.04'.

The docker *-ro bind mounts are dropped; instead only the release
tarball file name is exposed to the containers as *the* source, which
makes it unambiguous as to which sources are used.

The files related to RHEL builds are removed, as the RHEL build is
known to be broken and isn't being used.

* Makefile (DISTRIBUTIONS): New variable.
(make-docker-package-target): New function.
(docker/Dockerfile-snap): Adjust accordingly.
docker/*: Standardize to use hyphens instead of a mix of hyphens and
underscores.
* docker/Dockerfile_rhel_8: Delete file.
* docker/profile-seccomp-fedora_28.json: Delete file.
* scripts/make-packaging-target.py: Delete file.
* Jenkinsfile: Adjust accordingly.

Change-Id: I2dc42b489a54b177ad038398a9d23a2a7e3007ec