The tarfs image in cdhdinstall used to get padded out to an 18KB boundary
(the size of the read used to read it) with the alpha-specific image
creation. This got lost when moved to using common routines, so that
burning the cdhdinstall image onto a CDROM resulted in a failure to read
the last block of the tarfs image. Use the provided option to have the
image padded out to the full image size. It's more than actually needed,
but extra isn't likely to cause any problems.