MFC r258305, r258770, r259400, r259426:

r258305:
  Use the IMAGES variable to determine which image files to
  remove as part of 'make -C /usr/src/release clean'.

r258770 (hrs):
  Add NOPKG to disable pkg-stage.

r259400:
  Reduce disc1.iso size by 74MB by removing lib32 libraries,
  sendmail, and atf.

r259426:
  Add a pkg(8) repository configuration file for cdrom-based
  package installation.

  As part of the 'pkg-stage' target, copy the configuration
  file to the 'packages/repos/' directory on the DVD filesystem.

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.

NOTE:  WITH_DVD=1 is broken on stable/9, pending updates to the
pkg(7) code.  The 'dvdrom' target on stable/9 is broken as-is,
so there is no harm merging this change now.  I think.

Sponsored by:	The FreeBSD Foundation