MFC r313021, r313027, r313044:
r313021 (brd):
Force a few variables to be upper case when they are evaulated.
r313027 (brd):
Add a new vagrant-box target, to make it easier and less redundant to only produce the Vagrant box and not upload it.
r313044 (brd):
Add some comments with examples on how to build Vagrant images.
Sponsored by: The FreeBSD Foundation