openssh: limit read access to sshd_config

openssh: limit read access to sshd_config

Enhance security by limiting read access for /etc/sshd_config to user root as it
may reveal unsecure configurations.

Reading access is limited in the install append as the default value 0644 is
hardcoded in the openssh makefile and is not configurable. Therefore the
permissions are modified in the install append.

Signed-off-by: Louis Rannou <louis.rannou@non.se.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>