As man page says, installboot(8) is required only OpenFirmware 1 and 2
machines and even on such machines the root partition has to be at the
beginning of the disk to make installboot(8) work.
On OpenFirmware version 3 machines, the strategy provided by
installboot(8) doesn't work, and they need to have an extra
native partition (HFS or FAT16) which can be recognized by
the newer firmware to put a loadable bootloader.
If PART_A has some offset, we can assume the disk is partitioned
for a machine with OF3, which does't require installboot(8).
Eventually, we should provide an independent partitioning method
for OpenFirmware 3 machines (fdisk MBR partition + FAT16 partition
for ofwboot.xcf as OpenBSD does), but it may require some new API
to get OF version in sysinst.
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