tools/mkimage: Assume FDT image type for FIT images

When building a Flattened Image Tree (FIT) the image type needs to be
"flat_dt".  Commit 89a4d6b1 introduced a
regression which caused the user to need to specify the "-T flat_dt"
parameter on the command line when building a FIT image.  The "-T
flat_dt" parameter should not be needed and is at odds with the current
FIT image documentation.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>