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.

Sponsored by:	The FreeBSD Foundation
Use VMBASE in place of a hard-coded filename in the CLEANFILES list.

Sponsored by:	The FreeBSD Foundation