units: rename/rework systemd-boot-system-token.service → systemd-boot-random-seed...

units: rename/rework systemd-boot-system-token.service → systemd-boot-random-seed.service

This renames systemd-boot-system-token.service to
systemd-boot-random-seed.service and conditions it less strictly.

Previously, the job of the service was to write a "system token" EFI
variable if it was missing. It called "bootctl --graceful random-seed"
for that. With this change we condition it more liberally: instead of
calling it only when the "system token" EFI variable isn't set, we call
it whenever a boot loader interface compatible boot loader is used. This
means, previously it was invoked on the first boot only: now it is
invoked at every boot.

This doesn#t change the command that is invoked. That's because
previously already the "bootctl --graceful random-seed" did two things:
set the system token if not set yet *and* refresh the random seed in the
ESP. Previousy we put the focus on the former, now we shift the focus to
the latter.

With this simple change we can replace the logic
f913c784ad4c93894fd6cb2590738113dff5a694 added, but from a service that
can run much later and doesn't keep the ESP pinned.