nixos/installer/cd-dvd/iso-image: Allow booting on CHRP machines

[OPNA2608]

...& New World ROM Macs

Must be built on powerpc64-linux, cross is blocked by at least:

• rust-bindgen/rustPlatform.bindgenHook: Missing target triplet (& ABI) fixes from CC wrapper #473862

Needs:

• mesa: Only build OpenCL stuff on platforms where it works, fix build on ppc64 #456839
• linux: Adjust config to allow booting of installer ISO on ppc64 #486572

WIP.

• Under QEMU:
  • pseries,x-vof=off: Works after linux: Fix booting under qemu's IBM pSeries emulation #558934
  • mac99: Starts booting configuration, but not much more output after systemd[1]: Started Journal Service..
• nixos/modules/installer/cd-dvd/ppm2osbadgeicon.c is a custom tool I hacked together for customising the image displayed in the graphical boot selector on New World ROM Macs, see image above. The default image from GRUB's CHRP & tbxi scripts is the GNU logo. I wanted to differentiate the icon for the installer from the icon of the already-installed Debian system (which uses the GRUB default).
  If this is deemed useful, I can throw the code into a separate repo and package it so we don't need to vendor it.
  Edit: https://github.com/OPNA2608/netpbm2osbadgeicons, needs packaging if we want to use this.
  If this customisation is deemed too much, that's also fine.
• Related to the above: I have vendored a flake.ppm (pride flake logo), because the image produced by imagemagick on hardware has a tint added to the entire result that results in wrong colours (converting nix-snowflake.svg makes it look more like the old blue-purple colour scheme). Would like to drop this and instead generate it, but shipping weird colours…
  Edit: Should be fixed, was a bug in imagemagick. Was working around this with librsvg, need to undo this.
• CHRP entry point (/ppc/bootinfo.txt) is mostly copied from GRUB. Would like to eventually generate both the CHRP bootinfo.txt and the New World ROM Mac BootX file from an attrset with format-specific adjustments made to it (different SGML tags & formats used for the boot menu icon, different tag capitalisation requirements).
• Would like to rip some changes that cause rebuilds on other ISO targets out into an initial PR, then adjust this one so it's 0 rebuilds for all our existing installers.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • powerpc64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.