- Add SRC_FORCE_CHECKOUT configuration option to force svn to checkout
   the src/ tree into a directory that contains files/directories, such
   as a case where a custom kernel configuration file is specified.

 - Allow specification of multiple KERNCONFs to pass to 'make release'.

 - Move evaluation of NODOCS/NOPORTS earlier, and set based on how the
   release process expects these options to be evaluated.

 - Wrap KERNCONF specification in double quotes, and use 'eval' so multiple
   kernel configurations do not cause the build to fail in strange ways.

 - Set WITHOUT_X11 for the port build flags for the documentation toolchain
   build.  Also run 'clean distclean' targets during port build.

PR:		180192
MFC after:	3 days
Approved by:	kib (mentor, implicit)

This is a direct commit to stable/9.

Approved by:	kib (mentor, implicit)

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)