The call put_ssh_key assumes a destination file of ~/.ssh/authroized_keys on the DUT. Yet when an embedded system (such as OpenWrt) uses dropbear as the ssh server daemon, the authorized_keys file may be located elsewhere. For example, with OpenWrt, the authorized_keys file is found in /etc/dropbear/authorized_keys

An optional attribute such as dest_authorized_keys with a default value set to ~/.ssh/authorized_keys which can be overridden with an alternative filename for the DUT would be a practical solution.