openssh: add option to build sftp-server only

Lightweight ssh server implementations (Dropbear) usually lacks sftp
implementation but they can be used with one comming from openssh package.
With this option only sftp-server executable is built and installed into
/usr/libexec.
dropbear: add option to use sftp-server from openssh
Merge pull request #2 from xhpohanka/for-coretech

SFTP support is missing in current enviroment