device-qemu-ppc64le: support proper OF boot

Aelinrequested to merge
Aelin/pmaports:prep into master

This instructs pmbootstrap to build an image with two partitions: A PReP boot partition and the root partition. A separate /boot partition is not necessary on POWER VMs.

The PReP boot partition holds the bootloader ELF binary, in our case, this will be GRUB, which also needs to be configured, so we also enable the necessary option.

Edited by Aelin

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