Support systemd-boot random seed

[Conni2461]

Thanks for your work on this amazing project.

I've just successfully setup lanzaboote and enabled random seed using bootctl. After a reboot i noticed that random seed exists but isn't passed to the OS.

Random Seed:
 Passed to OS: no
 System Token: set
       Exists: yes

[nikstur]

Relates to #126 because it requires writing a "system secret" into the efivars.

[blitz]

Is there documentation on how the random seed is supposed to work?

[nikstur]

There is this article that explains it fairly well.

Edit: This https://github.com/systemd/systemd/commit/0be72218f1c90af5755ab40f94d047ee6864aea8 describes the current implementation.