make sure original genkernel/genkernel-next ebuilds provide `--firmware-src` and `--firmware-dst`

[pavlix]

We currently use a custom genkernel ebuild with a patch featuring --firmware-src and --firmware-dst options. It would be nice if we could actually work with original Gentoo packages.

• genkernel
  • custom package (from funtoo) already included
  • [ ] upstream and gentoo – https://bugs.gentoo.org/show_bug.cgi?id=600262
• genkernel-next
  • [x] create patch
  • [ ] upstream – https://github.com/Sabayon/genkernel-next/pull/41
  • [ ] gentoo