image: pass device blocksize to padjffs2

At the moment the padding steps are hardcoded. Especially images for
devices with a 4K sector size can be unnecessarily bloated using the
hardcoded padding steps.

It has been observed that 192Kb of padding was added to the image of a
4MB device, albeit due to the 4K sector size the minimum required extra
padding for the jffs2 rootfs_data is 20Kb.

In worst case it means that the image-size check could fail albeit
there is enough space for all selected packages

For device build code not exposing the blocksize, use the hardcoded
padding further on.

Signed-off-by: Mathias Kresin <dev@kresin.me>