Implement downstreamkernel_package

See #328 (closed)

I've built all changed packages (took a long time!) and they all still build except for linux-lg-bullhead and linux-motorola-athene which I doubt are caused by this MR (they both try to build Wireguard but the actual Wireguard source isn't included in the kernel sources, I wonder how the build for these packages ever worked?)

This MR should be tested on a few devices at least to make sure it works properly. Please also take extra care while reviewing the changes made in "custom conversions" because those weren't done automatically but manually.

Conversion patches I used for most packages: https://gist.github.com/z3ntu/cebe85fa15882e9b910e8638e4472688