update hammerhead to 5.2_rc3 and make freedreno work again

Thanks to @masneyb's amazing work, we can update the kernel to 5.2_r3. I've taken his patches, and created a separate mesa-freedreno package, so there is no conflict with other devices (mesa master is currently breaking lima etc.).

TODO:

• fix usb networking => #312 (closed)
• get logs, to see if it is really using freedreno instead of swrast (See "OpenGL vendor string" "freedreno" in Martijn's log below)
• fix initramfs hack (see last commit) to properly wait until whatever needed device appears before doing setup_mdev, instead of doing "sleep 1"

CC: @MartijnBraam, @pparent, @minlexx, @julian-goldsmith, @unrznbl, @bshah

I'm still pretty busy with blogging, would be great if you guys could take it from here.

Fixes #221 (closed).