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.
```

Start by reproducing the intermittent failure at GPIO.setup(io, GPIO.IN) on the reported BeagleBone image and examine the missing-file or permission condition shown in the traceback. Compare successful and failing runs to identify the timing-related difference; done means the failure is explained and the GPIO mode setup no longer raises this error under the reported conditions.

Written by the indexing model from the issue text.