Store the key on sshfs share:
Install sftp on openwrt:

ipkg install openssh-sftp-server

mount the share:

sshfs root@openwrt2:/mnt/nfs /mnt/tmp

unmount the share:

fusermount -u /mnt/tmp

ssh login without password
Create keys:

ssh-keygen -t rsa

use defaults to have key stored in /root/.ssh/id_rsa.pub

copy key to destination host:

cat /root/.ssh/id_rsa.pub | ssh root@openwrt2 'cat >> /etc/dropbear/authorized_keys'
ssh root@openwrt2
chmod 0600 /etc/dropbear/authorized_keys