mkbootimg: Add --dt parameter to specify DT image

New optional --dt parameter to specify a kernel device
tree image.

Change-Id: Ie29a11cbf4138426bfd19ae486d69a5fcbd8f442
mkbootimg: fixing the typecast issue

Change-Id: Ic375089ecce7aed3f9968e5e0b48e8ae0df66d83