Undefined name: true --> True

% `flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics`
```
./incubator-nuttx/tools/flash_writer.py:360:46: F821 undefined name 'true'
                    print("#", end="", flush=true)
                                             ^
1     F821 undefined name 'true'
1
```

tools/flash_writer.py

@@ -357,7 +357,7 @@ class TelnetDev:
                 if MAX_DOT_COUNT < cur_count:
                     cur_count = MAX_DOT_COUNT
                 for idx in range(cur_count - self.count):
-                    print("#", end="", flush=true)
+                    print("#", end="", flush=True)
                 self.count = cur_count
                 if self.count == MAX_DOT_COUNT:
                     print("\n")