Update the pkg-stage target to be more compatible with pkg-1.2:
- Add a release-dvd.conf pkg(8) configuration file to override
the default FreeBSD.conf configuration.
- Remove architecture-specific pkg-stage.conf files, consolidate,
and move their contents to scripts/pkg-stage.sh.
- Use 'pkg -vv' to determine the ABI, which is used as the
cache directory.
Prior to these changes, it would be possible for pkg-stage to fetch
conflicting binary packages from multiple repositories.
Tested against: head@r260522, stable/10@r260522
Sponsored by: The FreeBSD Foundation
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