build.7: Improve building pkgbase

The packages target was previously listed under kernel, move it to the
general source make targets listing. Add the update-packages target as
well. Adjust the release building targets with package in their names
to clarify they are unrelated to pkgbase. Add the OBJROOT and REPODIR
variables to ENVIRONMENT.

MFC after: 3 days
Reviewed by: ivy, markj (both previous versions)
Suggestions by: des, emaste, ivy, kevans, markj
Fixes: 1d26746cfd4a (Document the packages target)
Fixes: 0ac8aa55da1c (Add incremental packages)
Fixes: 26490d9b74f0 (allow update-packages for first)
Differential Revision: https://reviews.freebsd.org/D50289