Ensure compliance with freedesktop bootloader specification

[ikeydoherty]

Main ticket items:

• Write the version string for systemd-boot
• Write the machine-id string for systemd-boot
• Encode machine-id into target filenames on all systems

For natively booted systems, utilise the machine-id to ensure unique paths. This will ensure that CBM managed OSs can dual-boot with themselves (Dev/Production split)

For image mode builds, continue to use the current paths. This ensures that the first time CBM is run natively, it will perform a migration to the now-available machine-id filenames.

[ikeydoherty]

Sidenotes: We'll need to introduce some kind of FileCopier mechanism which can handle all of the migrations we do. This will also help us handle secure boot file assets, etc.