Add FilesSignature to packages

Initially it wasn't clear that we need signatures but it turns out it is going to help with one specific issue and that is: when someone either forgets to bump package or if change happened externally (trough OpenWrt configuration) that is not covered by bump that causes change of content of package. This would be covered by signature generated on list of files.

The updater issue: turris/updater/updater#313 (closed)