If the rootfs is read-only and the ssh keys are not available at system
start-up, the init script will generate ssh keys into /etc/ssh, thus
causing a 'read-only file system' error.
In order for Yocto based image to work correctly for read-only rootfs,
we use the following logic for openssh.
If the rootfs is read-only and there are pre-generated keys under /etc/ssh,
we use the pre-generated keys. Note the pre-generated keys are mainly for
debugging or development purpose.
If the rootfs is read-only and there are no pre-generated keys under
/etc/ssh, we use /var/run/ssh as the location for ssh keys. That is, at
system boot-up, the generated ssh keys will put into /var/run/ssh.
