pkgbase: Create two new rules for creating repo

[manu]

Summary

This adds two new rules named create-packages-kernel-repo and
create-packages-world-repo.
The goal of those rules is to create the {kernel,world} packages and
after that the repository.
It helps a lot for developing with pkgbase by adding the dev machine
repository created by those rules on top of the official pkgbase one.

Sponsored by: Beckhoff Automation GmbH & Co. KG

[emaste]

Do we have documentation that describes these targets? (i.e., where would an end user discover that these have been added?)

[manu]

In D43623#994732, @emaste wrote:

Do we have documentation that describes these targets? (i.e., where would an end user discover that these have been added?)

We have nothing on all pkgbase related targets, we probably should.