package/openssh: allow separate selection of client, server, keyutils

package/openssh: allow separate selection of client, server, keyutils

The openssh package comprises three separate entities: the SSH client, SSH
server, and some SSH key utilities. One may want the client but not the
server, the server but not the client, or maybe only the key utilities.

Add separate options for each entity and update the files installed on
target accordingly.

On an ARM Cortex-A53 configuration, size of stripped binaries are:

Client programs: 2213118 bytes (2161 KB)
usr/bin/ssh,657180
usr/bin/scp,99836
usr/bin/ssh-add,312800
usr/bin/ssh-agent,296428
usr/libexec/ssh-keysign,398908
usr/libexec/ssh-pkcs11-helper,292316
usr/bin/sftp,144992
usr/bin/ssh-copy-id,10658

Server programs: 806840 bytes (787 KB)
usr/libexec/sftp-server,112140
usr/sbin/sshd,694168
etc/init.d/S50sshd,532

Key utilities: 789648 bytes (771 KB)
usr/bin/ssh-keygen,398924
usr/bin/ssh-keyscan,390724

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>