paches/openwrt: add FilesSignature
This is variation on LinkSignature but it covers files. It should cover case when content of package changes. This can happen with OpenWrt configuration or just by mistake.
This also reworks original script for LinkSignature a bit. This is just to make both script consistent between each other.
Implements #257 (closed) and relates to turris/updater/updater#313 (closed).
Edited by Karel Koci