Adding systemd-boot options to example preseed.txt

["David Härdeman" <david@hardeman.nu>]

Hi,

now that experimental support for systemd-boot is in debian-installer,
it would be nice if the example preseed files could also document the
options to enable it:

The obvious place is:
https://salsa.debian.org/installer-team/installation-guide/-/blob/master/en/appendix/preseed.xml?ref_type=heads

But there's also other files that I'm not quite sure where they originate
from, like:
https://d-i.debian.org/manual/example-preseed.txt
https://www.debian.org/releases/trixie/example-preseed.txt

Regards,
David