Added usage prompt to flash utility

flash/main.c

@@ -17,6 +17,11 @@ struct opts
   size_t size;
 };
 
+static void usage(void)
+{
+    puts("stlinkv1 command line: ./flash {read|write} /dev/sgX path addr <size>");
+    puts("stlinkv2 command line: ./flash {read|write} path addr <size>");
+}
 
 static int get_opts(struct opts* o, int ac, char** av)
 {
@@ -75,6 +80,7 @@ int main(int ac, char** av)
   if (get_opts(&o, ac - 1, av + 1) == -1)
   {
     printf("invalid command line\n");
+    usage();
     goto on_error;
   }