Add a script and configuration files to fetch pre-built packages
from pkg.FreeBSD.org for inclusion on release medium (dvd1.iso).

The script sources ${.CURDIR}/${TARGET}/pkg-stage.conf, which sets
several environment variables, such as the pkg(8) ABI, PACKAGESITE,
PKG_DBDIR, and PKG_CACHEDIR.  PKG_CACHEDIR is set to the directory
on the release medium used by bsdconfig(8) (/packages/${ABI}).  ABI
is determined by output of 'make -C /usr/src/release -V REVISION'.
See pkg.conf(5) for descripton on other variables set here.

The list of packages to include are set within the configuration
file.

The script and configuration files are intended to be run by the
'make dvd' target within the release directory, and assume the
release is built within a chroot environment (such as by using
release.sh).

Relevant updates to release/Makefile will follow.

Sponsored by:	The FreeBSD Foundation