Include variant files into build directory for generic packages

[dappnodedev]

For generic packages, like https://github.com/dappnode/DAppNodePackage-lodestar-generic, we need to add the grafana dashboard and the prometheus targets to each of the variants directories, as their content will differ depending on the variant. The solution to this would be copying all the files included inside the variant directory into the build directory. Example:

/package_variants/mainnet/prometheus-targets.json should be included into build_lodestar.dnp.dappnode.eth_0.1.15/

Grafana dashboard could be common to all the variants, so it might be kept in the root of the repository