Add a build knob MAKE_DVD to control on a per-architecture basis whether

Add a build knob MAKE_DVD to control on a per-architecture basis whether
or not to build a tree used for the creation of a DVD image.  If that is
enabled set up a DVD tree by installing everything we normally install
to the individual CDROM trees into the one DVD tree.  The result is one
image with all the install bits, livefs bits, and doc bits suitable for
burning to a DVD instead of CDROM.

Enable building the DVD for amd64 and i386.

MFC after:	1 week