In its current form and shape, it creates images with the correct
partition contents and overall image layout. What it doesn't do
yet is actually scribble the metadata, such as partition tables
and boot code. This is where I'd like to leverage code from other
places, such as geom_part, and if possible/feasible.
Also, the utility should be a little bit smarter about files and
pipes so that we create temporary files only when needed (e.g.
when we don't know the size of a partition's contents in advance
while wrting the image to stdout).
