I'm trying to make an embedded device, and I would like to start with the minimal setup as possible. Are there some units that are required by systemd? I ask because systemd is complaining about a missing rescue.target unit, but I don't list rescue.target as a dependency of any other unit. However, grep tells me that /usr/lib/systemd/systemd contains the "rescue.target" string, which worries me. Is there a list of all the units that are required, even though no other unit lists them?