libre: Add linux-libre-vanilla
    
    This new kernel enables to test if maintaining a kernel without
    additional patches is easier and faster:
    - Not having to refresh patches makes it faster to update the
      kernel. And the procedure to liberate the Arch Linux patches is also
      not documented (though with trial and errors someone can relatively
      easy manage to do it, but that's time consuming).
    - Some of the patches are never upstreamed so they stay there for
      years. This also means that what work in Parabola is broken in other
      FSDG compliant distributions because they don't use these patches.
    
    In addition this kernel also enable users to test their hardware with
    a stock linux-libre kernel. In the past the armv7h Parabola kernel
    carried out patches to make capes work on the Beaglebone Single Board
    Computers.
    
    This made me think that this feature was upstream because it worked in
    Parabola. The patches to do that were removed at some point so it
    stopped working. So if I started relying on it it would have been a
    very bad idea as it would have stopped working without even a warning.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>