Components like base.txz and ports.txz are called distributions in the
installer, and with the introduction of pkgbase we will start dealing
with normal pkg packages in the installer. Rename EXTRA_PACKAGES to
DISTRIBUTIONS, and move base.txz and kernel.txz to that list.
This introduces no functional change but is a small cleanup in advance
of some pkgbase experimentation.
Reviewed by: cperciva
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43544