reproducible openwrt: add snapshot build script

The script downloads all buildinfo and checksums from the OpenWrt
download server and tries to rebuild them. Once done the upstream
checksums are used to verify the newly created files.

A website based on templates-mustache/openwrt/ is rendered.

Use locally installed diffoscope.

Store created and origin packages in database.

Signed-off-by: Paul Spooren <mail@aparcar.org>