ci: build package for Turris

Thanks for all the help, @jpavlinec !

I placed the packaging files into distro/turris. If there are any updates to them, please don't hesitate to open a merge request :)

The only concern I have is we can only do builds from this public repo with the current setup. Would it be possible to convince the openwrt Makefile to use a specific directory, or a tarball instead of providing COMMIT_SHA, which is then downloaded from upstream?