mkimage: fix option parsing segfault

getopt_long() expects a NULL-terminated list of structures. The current
list in mkimage does not have a zero-filled structure at the end, which
can cause getopt_long() to walk past the end of the array when passing
an unknown option, causing a segmentation fault.

As a reproducer, the following command causes a segmentation fault
(tested in Debian 12):

    mkimage --foobar

Signed-off-by: Carlos López <carlos.lopezr4096@gmail.com>

tools/mkimage.c

@@ -196,6 +196,7 @@ static const struct option longopts[] = {
 	{ "verbose", no_argument, NULL, 'v' },
 	{ "version", no_argument, NULL, 'V' },
 	{ "xip", no_argument, NULL, 'x' },
+	{ /* sentinel */ },
 };
 
 static void process_args(int argc, char **argv)