MFreleng10 r259582 (reverse), MFstable10 r259491, r259492, r260781:

r259582 (reverted):
Set PACKAGESITE to 'release/0' for the pkg-stage target to pull
the release set of packages. (Required to eliminate conflicts.)

r259491:
Prevent release build errors found during snapshot builds where if
NOPORTS=1, pkg-stage.sh cannot build the ports-mgmt/pkg port if
WITH_DVD=1.

r259492:
Add NOPKG to disable pkg-stage.

r260781:
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.

A change local to releng/10.0 sets the package fetch URL to
'release/0'.

Approved by: re (delphij)
Sponsored by: The FreeBSD Foundation

MFC r260772:
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

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