dropbear make pub key

4 Steps:
a) Generate private rsa key in a file: dropbearkey -t rsa -f /etc/dropbear/dropbear_rsa_host_key
b) Generate public rsa key in a file: dropbearkey -y -f /etc/dropbear/dropbear_rsa_host_key | grep ssh-rsa > /tmp/dropbear_rsa_host_key.pub
c) Copy the public key to the server: scp /tmp/dropbear_rsa_host_key.pub haritz@192.168.111.253:/home/haritz/.ssh/authorized_keys
d) scp or ssh indicating the path of the 'identity_file': ssh -i /etc/dropbear/dropbear_rsa_host_key haritz@192.168.111.253
The identity_file is needed (at least in my case).