openssh: Add PACKAGECONFIG option to customize sshd mode

openssh: Add PACKAGECONFIG option to customize sshd mode

Add systemd-sshd-socket-mode PACKAGECONFIG option to choose installing
sshd.socket and systemd-sshd-service-mode PACKAGECONFIG option to choose
installing sshd.service.

The systemd-sshd-socket-mode PACKAGECONFIG option is enabled by default
and user can choose the above two PACKAGECONFIG option to customize the
sshd mode.

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>