The imgtools work as a set of layers so that we can have multiple
formats.  The IMG_INFO that is returned by img_open is the highest
layer.  It corresponds to the last name in the list (i.e. raid in
"split,raid").  Any request that it recieves it calculates where
the data should exist and then passes that address to the next
layer.  The lowest layer knows where the actual image files are and
reads the data.

There are some limitations:
- the split layer cannot have any layers below it. 
