Main change in building images is that an empty 4096 byte block is
prepended to raw image before compression so that upon decompression
the header can be written without having to shuffle around decompressed
image.
--just-choose will print information about which partition would be
chosen to install a rootfs image.
--skip-sha will avoid checking the sha256 sum of the image file. This
is mainly meant for use by the installer since the sum has already
been confirmed
