K64F Support - Max Image Sectors Configuration

[AGlass0fMilk]

The K64F support is currently a bit broken. The maximum number of sectors must be configured if the user inserts an SD card with a lot of small sectors.

Additionally, it may be required to add a BufferedBlockDevice on top of the SlicedBlockDevice that is returned by get_secondary_bd() in this example -- this would be a good way to ensure compatibility regardless of the underlying BlockDevice.