openssh: Allow enable/disable of rng-tools recommendation on sshd

We are adding a new PACKAGECONFIG option ('rng-tools') to control if we
wish the openssh-sshd to RRECOMMENDS the 'rng-tools' package. We are
enabling it by default so there is no behavior change.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>