pkgbase: Create two new rules for creating repo

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.

Differential Revision: https://reviews.freebsd.org/D43623
Reviewed by: bapt, emaste
Sponsored by: Beckhoff Automation GmbH & Co. KG