Clarify ISO padding and partition table support

[mika]

Within grml-live we run this:

      # pad the output ISO to multiples of 256 KiB for partition table support
      siz=$($getfilesize "${ISO_OUTPUT}/${ISO_NAME}")
      cyls=$((siz / 512 / 32 / 16 + 1))   # C=$cyls H=16 S=32
      siz=$((cyls * 16 * 32 * 512))   # size after padding
      dd if=/dev/zero bs=1 count=1 seek=$((siz - 1)) \
         of="${ISO_OUTPUT}/${ISO_NAME}" 2>/dev/null

Now that we're dropping manifold and grub boot (see #152), let's find out whether this code is still needed nowadays (isohybrid might do the right™ thing already?).

Also would be nice to check whether we can actually partition our dd-ed ISO images, so a USB device with 4GB doesn't end up only with ~1GB ISO stuff and remaining 3GB of disk space can't be used.

[mika]

Depending on outcome, maybe also drop HYBRID_METHOD='disable' and always run isohbyrid and don't support disabling it.

[mika]

Ah, the partition table padding also takes place in remaster/grml-live-remaster, if we're dropping the padding don't forget to remove it also from there.