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.

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

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

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.

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

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

Fix UFS label for the root filesystem.

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

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.

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

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.

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.

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

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.

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

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