MFC r251652, r251811, r252101:

MFC r251652, r251811, r252101:

r251652:
  Add new script to run release builds.  The script is heavily based upon
  the release/generate-release.sh script by nwhitehorn.

  This script can use optional configuration file to override defaults,
  making running multiple serialzied builds with different specific
  configurations or architecture-specific tunings quite easy.

  Sample overrideable options are included in the release.conf.sample
  file.

r251811:
  Fix OSVERSION variable within PBUILD_FLAGS.

r252101:
  If MAKE_CONF and/or SRC_CONF are set and not character devices,
  copy to chroot to use with release build.

Approved by:	kib (mentor, implicit)

git-svn-id: svn://svn.freebsd.org/base/stable/9@252224 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f