disk-label: increase max PREP boot image size to 64MB

The disk-label package limits the maximum PREP boot image size
to 32 MB. Recent AIX boot images generated by bosboot can exceed
this limit, causing the firmware to fail to load the complete
image and preventing the system from booting.

Increase the maximum supported PREP boot image size from 32 MB
to 64 MB.

Tested with QEMU pSeries and AIX using boot images up to 64 MB.

Signed-off-by: Dasipogu Naga Sreenu <Dasipogu.Naga.Sreenu@ibm.com>
Signed-off-by: Thomas Huth <th.huth@posteo.eu>