As with many things, we have scenarios where we don't want to boot on a
disk / bootloader and also we don't want to boot directly.
Sometimes, we want to boot through an OptionROM of our NIC, e.g. netboot
scenarios or let the firmware decide something, e.g. UEFI PXE (or even
UEFI OptionROM!).
This is composed of:
- `directBoot.enable`: whether to direct boot or not
- `directBoot.initrd`: enable overriding the
`config.system.build.initialRamdisk` defaults, useful for
netbootRamdisk for example.
This makes it possible.
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