oeqa/runtime/scp: requires openssh-sftp-server

oeqa/runtime/scp: requires openssh-sftp-server

The SCP protocol was deprecated in favor of the SFTP.
For the legacy SCP protocol scp should be run with "-O".
Instead of adding "-O" on the scp_options ssh oeqa we can
require the openssh-sftp-server to be instaled on the target.

This way the test will work more deterministic regardless of
the host machine client used.
For the old fashion clients still using legacy SCP protocol
the openssh-sshd server will be used, for the new ones using
the SFTP the openssh-sftp-server will be picked.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>