This command shows which areas of the flash memory are currently
not in use. When a block contains non-erased contents it is considered
in use. Since it is possible to force an image to be loaded at a
particular flash location, this command can be used to check whether
that location is in use by any other image.
Note: There is currently no cross-checking between actual flash
contents and the FIS directory, which mans that there could be a
segment of flash which is not erased that does not correspond to a
named image, or vice-versa.
