build: do not upload dst=''

On Ruby Capstanfile, it trying to upload directory named '' then causes abort.
https://gist.github.com/syuu1228/4d9c6c703d5097072253

This patch skips uploading the directory from capstan.

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>