New module to handle firstboot detection in the initrd

[Vogtinator]

Look at /etc/machine-id in the /sysroot, similar to what ConditionFirstBoot in the real root would do. If firstboot is detected, start firstboot.target.

TODO:

• [x] Import better x-initrd.mount handling into firstboot-detect
• [x] Remove 755 on firstboot-detect.sh
• [x] Only install the combustion module if no /etc/machine-id on the host
• [x] Adjust README.md
• [x] Make ignition use this, or at least compatible. Currently there's an ordering dep between ignition-fetch.service/network.target/firstboot-detect.service

[Vogtinator]

The firstboot-detect.sh script looks as horrible as expected

Horrible? I think it turned out fine actually...