mkimage: do not fail if there is no print_header function

Commit 253c60a5 breaks the exit value of 'mkimage -T rkimage'
and print the following  error:
  mkimage: Can't print header for Rockchip Boot Image support: Success

It is not a failure to not print headers, so just display the warning message,
and finish the function properly.
Signed-off-by: NGuillaume GARDET <guillaume.gardet@free.fr>

Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>

tools/mkimage.c

@@ -588,9 +588,8 @@ int main(int argc, char **argv)
 	if (tparams->print_header)
 		tparams->print_header (ptr);
 	else {
-		fprintf (stderr, "%s: Can't print header for %s: %s\n",
-			params.cmdname, tparams->name, strerror(errno));
-		exit (EXIT_FAILURE);
+		fprintf (stderr, "%s: Can't print header for %s\n",
+			params.cmdname, tparams->name);
 	}
 
 	(void) munmap((void *)ptr, sbuf.st_size);