tos3to4: add SSH daemon config migration

In Turris OS 3.x there was UCI config and integration for sshd. Now there is just standard config. Simple fix is to move original generated one to appropriate location.

This move only happens if configuration of sshd in UCI is not the default one. The second check is if target file exists and that is just to make sure but it should always be there.

Thanks to David Hopfmueller to pointing this out.

Fixes #622 (closed)