release: Enable installing packages as a non-root user

Provide variables, PKG_REPOS_DIR and PKG_REPO_NAME, to allow the user to
configure the source package repository. Configure pkg to emit added
files to a metalog which gets concatenated with the main metalog
immediately before we build the root filesystem.

Reviewed by: cperciva, emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52453