fixed a failed check for MMC device when re-flashing a BBB

This check failed on a BBB I had that had previously been flashed with Ubuntu 13.04 to its MMC. Here was what the grep returned before my change:

```/dev/mmcblk1:
/dev/mmcblk1boot1:
/dev/mmcblk1boot0:```

This obviously failed the check to see if the grep results matched the provided MMC device.