dtc: Change exit code for usage message

If dtc's command line arguments are invalid, it prints a usage message
and returns exit code 2.  That's the same exit code as for a failed
check, which is potentially confusing if running dtc from an automated
harness.  Therefore this patch changes the usage exit code to 3.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc.c

@@ -106,7 +106,7 @@ static void  __attribute__ ((noreturn)) usage(void)
 	fprintf(stderr, "\t\tForce - try to produce output even if the input tree has errors\n");
 	fprintf(stderr, "\t-v\n");
 	fprintf(stderr, "\t\tPrint DTC version and exit\n");
-	exit(2);
+	exit(3);
 }
 
 int main(int argc, char *argv[])