release: Ship firmware from kmods repo on DVD

The packages for X.Y-RELEASE are a snapshot of the quarterly branch
for stable/X, i.e. typically built on X.(Y-1)-RELEASE. (The case of
Y=0 is an exception for obvious reasons.) This works for most ports
but not for kernel modules, which may need to be built on the release
in question; this is why we have a separate "kmods" repository.

Make sure that we put the packages from the kmods repository onto the
mini-pkg-repo on the DVD rather than shipping unusable firmware.

Reviewed by: bz
MFC after: 1 week
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D52581

(cherry picked from commit bda8028146694ee490543b35e3349e060936fde4)