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.

Sponsored by:	The FreeBSD Foundation
After evaluating WITH_VMIMAGES is non-empty, ensure
the mk-vmimage.sh script exists before running it.

Sponsored by:	The FreeBSD Foundation
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.

Sponsored by:	The FreeBSD Foundation
Document the new 'vm-image' target, and associated release.conf
variables.

Sponsored by:	The FreeBSD Foundation
Remove two stray comments added during the initial iterations
of testing, no longer needed.

Sponsored by:	The FreeBSD Foundation
Reintegrate head@r272414

Sponsored by:	The FreeBSD Foundation
Remove the first argument to panic(), which was initially
intended to be the exit code, however when a non-zero exit
code was returned to release/Makefile, this would prevent
any remaining (and possibly successful) stages from being
attempted.

Sponsored by:	The FreeBSD Foundation
If the vm-base target fails, prevent the vm-image target
from being run since it cannot possibly succeed.

Sponsored by:	The FreeBSD Foundation
Add /usr/local/bin and /usr/local/sbin to PATH, needed
if third-party software needs to use utilities outside
of the base system during post-install stages (indexinfo
is one culprit).

Sponsored by:	The FreeBSD Foundation
Add prototypes for build-specific tuning:

 - vm_prebuild_setup():
	Steps to run prior to creating the file-backed
	disk image.

 - vm_setup():
	Steps to run while the file-backed disk image
	is mounted.

 - vm_postbuild_setup():
	Steps to run after the file-backed disk image
	is dismounted.

The intention is to override as necessary via build
configuration files, as needed.

Sponsored by:	The FreeBSD Foundation
Allow mk-vmimage.sh to accept a configuration file as
an argument to '-c'.

Sponsored by:	The FreeBSD Foundation
Reference vm_prebuild_setup(), vm_setup(), and
vm_postbuild_setup() functions.

Sponsored by:	The FreeBSD Foundation
Revert r272793, r272794, r272795:

  Remove implementation of vm_prebuild_setup(),
  vm_setup(), and vm_postbuild_setup().

  It does not scale well, and I am not happy with
  their implementation.

Sponsored by:	The FreeBSD Foundation
Add a separate make(1) target to release/Makefile to
build FreeBSD virtual machine disk images for use on
the Microsoft Azure service.

For now, this target is not directly connected to the
build, however can be manually invoked.

The 'vm-azure' target invokes {amd64,i386}/mk-azure.sh,
which does the heavy lifting to produce proper VHDs.
mk-azure.sh uses a configuration file, defaulting to
tools/azure.conf if otherwise unset.

Sponsored by:	The FreeBSD Foundation
Merge the following from ^/projects/release-vmimage:
r273076, r273077, r273079, r273095:

r273076:
  Add a separate make(1) target to release/Makefile to
  build FreeBSD virtual machine disk images for use on
  the Microsoft Azure service.

  For now, this target is not directly connected to the
  build, however can be manually invoked.

  The 'vm-azure' target invokes {amd64,i386}/mk-azure.sh,
  which does the heavy lifting to produce proper VHDs.
  mk-azure.sh uses a configuration file, defaulting to
  tools/azure.conf if otherwise unset.

r273077:
  Clear VM_RC_LIST.

r273079:
  Fix signal list to trigger umount(8).

r273095:
  Output an informational message when mkimg(1) runs, so it
  does not appear that the process has stopped while waiting
  for a 'y/n' response when waagent is deprovisioned.

Tested on:	releng/10.1@r272876
MFC after:	3 days
X-MFC-10.1:	yes
Sponsored by:	The FreeBSD Foundation