Clarify the actual reason machine-id can't be truncated earlier

Even after fixing https://github.com/systemd/systemd/issues/11925

 we
would still have issues by truncating the /etc/machine-id earlies, as
the actual problem is that the systemd-boot tools rely on it being set
for dual-boot scenarios:
* to avoid conflicts when creating entries, see the `90-loaderentry` kernel
  install trigger
* to set the entries for the currently booted installation as default in
  the loader.conf generated by `bootctl install`

In our image this is not useful, as the actual machine-id is supposed to be
uniquely generated on the first boot. However the impact is negligible, as
things still work albeit the code used to potentially disambiguate entries
doesn't match a real machine-id.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>