Merge the following revisions from ^/projects/release-vmimage:

r272234, r272236, r272262, r272264, r272269, r272271, r272272,
r272277, r272279, r272376, r272380, r272381, r272392, r272234,
r272412:

r272234:
  Initial commit to include virtual machine images as part
  of the FreeBSD release builds.

  This adds a make(1) environment variable requirement,
  WITH_VMIMAGES, which triggers the virtual machine image
  targets when not defined to an empty value.

  Relevant user-driven variables include:

   o VMFORMATS:	The virtual machine image formats to create.
     Valid formats are provided by running 'mkimg --formats'

   o VMSIZE:	The size of the resulting virtual machine
     image.  Typical compression is roughly 140Mb, regardless
     of the target size (10GB, 15GB, 20GB, 40GB sizes have been
     tested with the same result).

   o VMBASE:	The prefix of the virtual machine disk images.
     The VMBASE make(1) environment variable is suffixed with
     each format in VMFORMATS for each individual disk image, as
     well as '.img' for the source UFS filesystem passed to
     mkimg(1).

  This also includes a new script, mk-vmimage.sh, based on how
  the VM images for 10.0-RELEASE, 9.3-RELEASE, and 10.1-RELEASE
  were created (mk-vmimage.sh in ^/user/gjb/thermite/).

  With the order in which the stages need to occur, as well as
  sanity-checking error cases, it makes much more sense to
  execute a shell script called from make(1), using env(1) to
  set specific parameters for the target image than it does to
  do this in make(1) directly.

r272236:
  Use VMBASE in place of a hard-coded filename in the CLEANFILES
  list.

r272262:
  Remove a 'set -x' that snuck in during testing.

r272264:
  release/Makefile:
    Connect the virtual machine image build to the release
    target if WITH_VMIMAGES is set to a non-empty value.

  release/release.sh:
    Add WITH_VMIMAGES to RELEASE_RMAKEFLAGS.

  release/release.conf.sample:
    Add commented entries for tuning the release build if the
    WITH_VMIMAGES make(1) environment variable is set to
    a non-empty value.

r272269:
  release/Makefile:
    Include .OBJDIR in DESTDIR in the vm-base target.

  release/release.sh:
    Provide the full path to mddev.

r272271:
  Fix UFS label for the root filesystem.

r272272:
  Remove comments left in accidentally while testing, so the
  VM /etc/fstab is actually created.

r272277:
  Remove the UFS label from the root filesystem since it is added
  by mkimg(1) as a gpt label, consistent with the fstab(5) entry.

r272279:
  Comment cleanup in panic() message when mkimg(1) does not support
  the requested disk image format.

r272376:
  Separate release/scripts/mk-vmimage.sh to machine-specific
  scripts, making it possible to mimic the functionality for
  non-x86 targets.

  Move echo output if MAKEFLAGS is empty outside of usage().

  Remove TARGET/TARGET_ARCH evaluation.

r272380:
  Avoid using env(1) to set values passed to mk-vmimage.sh,
  and instead pass the values as arguments to the script,
  making it easier to run this by hand, without 'make release'.

  Add usage_vm_base() and usage_vm_image() usage helpers.

r272381:
  After evaluating WITH_VMIMAGES is non-empty, ensure
  the mk-vmimage.sh script exists before running it.

r272392:
  Add WITH_COMPRESSED_VMIMAGES variable, which when set enables
  xz(1) compression of the virtual machine images.

  This is intentionally separate to allow more fine-grained
  tuning over which images are compressed, especially in cases
  where compressing 20GB sparse images can take hours.

r272412:
  Document the new 'vm-image' target, and associated release.conf
  variables.

r272413:
  Remove two stray comments added during the initial iterations
  of testing, no longer needed.