system/permissions: make /root group+others non-writable

Upon logging, dropbear whines if /root is group- or others-writable, and
key-based authentication is attempted, reverting to password-based
authentication:
    dropbear[149]: /root must be owned by user or root, and not writable by others
    dropbear[149]: Password auth succeeded for 'root' from 192.168.127.35:41566

On my system, /root was 770. Changing to 700 fixed the issue.

Having /root 700 is a good idea, anyway.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>