units: condition out a few services in the initrd

units: condition out a few services in the initrd

Let's make our units more robust to being added to an initrd:

1. systemd-boot-update only makes sense if sd-boot is available in /usr/
   to copy into the ESP. This is generally not the case in initrds, and
   even if it was, we shouldn't update the ESP from the initrd, but from
   the host instead.

2. The rfkill services save/restore rfkill state, but that information
   is only available once /var/ is mounted, which generally happens
   after the initrd transition.

3. utmp management is partly in /var/, and legacy anyway, hence don't
   bother with it in the initrd.