dumpimage: add 'T' option to explicitly set the image type

Some image types, like "KeyStone GP", do not have magic numbers to
distinguish them from other image types. Thus, the automatic image
type discovery does not work correctly.

This patch also fix some integer type mismatches.

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>