release: Add pkg .conf to METALOG on dvd

We're correctly recording all of the packages in the dvd METALOG file,
but if we don't record ./packages/repos/FreeBSD_install_cdrom.conf then
users won't be able to install them very easily.

Approved by: re (cperciva)
Reviewed by: markj
Reported by: Lars Tunkrans
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D53199

(cherry picked from commit 2b5c62494df95cdb844b5f6f852412f2178f66dd)
(cherry picked from commit 52ea1ca18a0674e5085d6d0ddbc3c13bf0850429)