release: rework distributions list

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