pkgbase: Add a FreeBSD-pkg-bootstrap package

And put pkg and its keys in it.
It's easier for small image to depend on this package rather than the
larger utilities one.

MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33458

(cherry picked from commit ec0ea6efa1ad229d75c394c1a9b9cac33af2b1d3)