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