Fixed ofwboot and sysinst

Hi ofppc-users,

in the last weeks I fixed MBR support in ofwboot and disksubr.c.
Also unlabeled disks (CDs) are recognized again.

My goal was to ensure that setting up a fresh system with sysinst,
booted from CD, is working. So I made some fixes to sysinst too.

This seems to work fine with my Pegasos2 now. So ofppc is ready for 6.0.

Nevertheless I would be very interested if the installation procedure
is working similarly well on other ofppc systems. Please tell me when
there are any problem left!

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-- 
Frank Wille