tools/mkimage: fix compiler warnings, use "const"

This fixes some compiler warnings:
tools/default_image.c:141: warning: initialization from incompatible pointer type
tools/fit_image.c:202: warning: initialization from incompatible pointer type
and changes to code to use "const" attributes in a few places where
it's appropriate.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/tools/mkimage.h b/tools/mkimage.h
index 120970f..96f2ef8 100644
--- a/tools/mkimage.h
+++ b/tools/mkimage.h
@@ -104,7 +104,7 @@
 	 */
 	int (*verify_header) (unsigned char *, int, struct mkimage_params *);
 	/* Prints image information abstracting from image header */
-	void (*print_header) (void *);
+	void (*print_header) (const void *);
 	/*
 	 * The header or image contents need to be set as per image type to
 	 * be generated using this callback function.