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

This MR changes increases the min./max. size of the HFS 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).