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

MFC after: 3 days
X-Insta-MFC: possibly
Sponsored by: The FreeBSD Foundation