imds-generator: replace static Condition=initrd by a check in the generator

imds-generator: replace static Condition=initrd by a check in the generator

After looking at the unit, I'm not sure if systemd-imds-import.service
is supposed to run in the host system or not. But if it is supposed to
only run in the initrd, then the static condition in the unit file gives
as the worst behaviour: the generator does not do any checks if we are
in the initrd or not, and if it enabled the unit, it'll influence the
transaction ordering (possibly causing loops or additional work) and
then the unit will be unconditionally skipped. So replace the static
condition by a check in the generator. If the user specifies
systemd.imds.import on the commandline, it'll be honoured also in the
host.