Secure boot for devices which don't have hardware security element

I understand that Root of Trust is necessary for implementing a secure boot on a device. Root of Trust is strong and trust worthy if this comes from hardware security elements like HSM/TPM/.. So for devices which are not having hardware security elements, is there is way to implement Root of Trust in software? what is the right/best practice of implementing RoT and hence secure boot on such devices?
http://dlvr.it/RzZZ4v