dtdiff: Use input format dtb for dtbo files

The file ending .dtbo is typically used for device tree overlays. These
are in the dtb input format, too. So assume this input format for *.dtbo
as is already done for *.dtb.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtdiff

@@ -17,7 +17,7 @@ source_and_sort () {
 	    *.dts)
 		IFORMAT=dts
 		;;
-	    *.dtb)
+	    *.dtb|*.dtbo)
 		IFORMAT=dtb
 		;;
 	esac