Support the s390x bootloader zipl (z Initial Program Loader). We
supply the parameters for the kernel+initrd as well es the target,
i.e. the boot partition where the bootmap is creating, the device,
here called 'targetbase', to install the bootloader on, including
parameters describing the device (type, blocksize) and also the
offset of the partition containing the target from the start of
device (in sectors).
The kernel and initrd are found via the bootloader entry, ignoring
the rescue kernel.
Since zipl needs the device as well as access to the boot partition
the image is bound to a loopback device. Also keep the filesystem
tree mounted during the execution of the zipl installation.
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