Improvements

Improved file naming scheme for SD cards in FAT mode: now uses "VOLxx.PO" filenames for volumes (xx=00-0F). Only the first 5 characters ("VOLxx") and the file extension (".PO") matters. So you can keep your files on the SD cards organized: just keep the original filename (e.g. "TotalReplay.po") and add a "VOLxx" prefix. The DAN][ card recognizes the file according to the prefix (and shows it as volume number xx).
Example SD card root directory using the new scheme:
```
  VOL00_TotalReplay.po
  VOL01_InstantReplay.po
  VOL02_Apple2Desktop.po
  VOL03.po
  VOL04.po
  VOL05.po
  ...
  VOL0F_foo_bar.po
```
Note: For backward-compatibility the old file naming scheme "BLKDEVxx.PO" is also still supported. However, this scheme does not support longer file names (no characters are ignored, filenames have to match exactly). Also, you cannot mix the naming schemes: not on a single SD card, nor on two SD cards plugged at the same time.

Hardware Bring-up/LED support: To help with building/testing new boards, the LEDs now provide better feedback. When no SD cards are detected but the card is otherwise OK, then the two SD slot LEDs show an alternating LED blink pattern for 3 seconds (when powering the card).

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