extract_flash.c will fail for large programs on dual-256K flash boards

[tomlogic]

IIRC, the memory layout of dual flash boards places the UserBlock and System ID Block at the top of the first flash. I think this was to allow for storing data on the second flash, or possibly an alternate firmware image (downloader on first flash, application on second flash).

The current version of Samples/extract_flash.c doesn't take that mapping into account, and won't be able to dump a large firmware image that skips over the UserBlock and System ID Block.