ValueError: Set gpio mode failed, missing file or invalid permissions.

Still have the same issue as multiple users reported, though I'm running a quite recent BBB image U-boot. I also do not experience it every time, it seems to be some timing issue or something?

```
Aug  9 06:51:38 :     [ ... ]
Aug  9 06:51:38 :     GPIO.setup(io, GPIO.IN)
Aug  9 06:51:38 : ValueError: Set gpio mode failed, missing file or invalid permissions.
```