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>
diff --git a/README b/README
index fefa71c..a11e74a 100644
--- a/README
+++ b/README
@@ -5899,9 +5899,10 @@
 option). Given an image built by mkimage, the dumpimage extracts a "data file"
 from the image:
 
-	tools/dumpimage -i image -p position data_file
-	  -i ==> extract from the 'image' a specific 'data_file', \
-	   indexed by 'position'
+	tools/dumpimage -i image -T type -p position data_file
+	  -i ==> extract from the 'image' a specific 'data_file'
+	  -T ==> set image type to 'type'
+	  -p ==> 'position' (starting at 0) of the 'data_file' inside the 'image'
 
 
 Installing a Linux Image: