WIP:[powerpc/ppc64] Prepare for GRUB installations on NewWorld Power Macs

This MR changes the FS of the OF bootstrap partition from HFS to FAT16 and increases the minimal size of the bootstrap partitions for NewWorld Power Macs to prepare for future GRUB installations.

A size between 10 and 20 MB was chosen to also support future GRUB installations that might exceed 10 MB, e.g. at the moment on one of my G5 Power Macs a GRUB installation already needs more than 8 MB, with the localizations accounting for the biggest part:

root@powermac-g5:/boot/grub# tree -a -h --du
.
├── [ 16K]  BootX
├── [2.3M]  fonts
│   └── [...]
├── [ 80K]  grub
├── [9.8K]  grub.cfg
├── [1.0K]  grubenv
├── [4.1M]  locale
│   └── [...]
├── [   0]  mach_kernel
├── [2.8K]  ofboot.b
├── [1.7M]  powerpc-ieee1275
│   └── [...]
├── [ 97K]  System
│   └── [ 97K]  Library
│       └── [ 97K]  CoreServices
│           ├── [ 16K]  BootX
│           ├── [1.9K]  .disk_label
│           ├── [  14]  .disk_label.contentDetails
│           ├── [ 79K]  grub.elf
│           └── [ 240]  SystemVersion.plist
├── [194K]  yaboot
└── [ 661]  yaboot.conf

 8.4M used in 6 directories, 263 files

The sizes of the localizations range from 18K to 190K, so the size of a GRUB installation can grow quickly with each new localization.

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Tested successfully on Mac mini G4 and Power Mac G5 (11,2).