truncate boot_out.txt if it's long

Now this boot.py:
```py
for i in range(1000):
    print(i)
```
creates a 512-byte boot_out.txt that ends
```
88
89
...
```

supervisor/shared/micropython.c

@@ -60,8 +60,14 @@ void mp_hal_stdout_tx_strn(const char *str, size_t len) {
 
     #ifdef CIRCUITPY_BOOT_OUTPUT_FILE
     if (boot_output != NULL) {
+        // Ensure boot_out.txt is capped at 1 filesystem block and ends with a newline
+        if (len + boot_output->len > 508) {
+            vstr_add_str(boot_output, "...\n");
+            boot_output = NULL;
+        } else {
             vstr_add_strn(boot_output, str, len);
         }
+    }
     #endif
 
     serial_write_substring(str, len);