The fact that it's a "single board" computer, specifically, is mildly irrelevant, imo; just follow standard backup practices. The only way the type of computer really comes into question is whether or not it has adequate resources to run whatever backup solution that you choose. For my usecase, Borg works great, but choose whatever solution fits your requirements. The "simplest", and lightest solution is probably rsync, but that may leave a lot to be desired.