core: delay logging the taint string until after basic.target is reached (#7935)

core: delay logging the taint string until after basic.target is reached (#7935)

This happens to be almost the same moment as when we send READY=1 in the user
instance, but the logic is slightly different, since we log taint when
basic.target is reached in the system manager, but we send the notification
only in the user manager. So add a separate flag for this and propagate it
across reloads.

Fixes #7683.