uboot4extlinux-sunxi: Fix u-boot FSDG compliance
    
    U-boot contains some nonfree software, for instance nonfree microcode
    in arch/x86/dts/microcode/, so this commit removes that.
    
    This commit also removes instructions to install nonfree
    software. While these instructions were not shipped in any of the
    binary pakcages generated by this PKGBUILD, it's still a good idea to
    also clean that up in the long run:
    - Parabola has also PKGBUILDs for other SOCs, and ideally we could
      expand to cover as many computers as possible if the computers are
      supported in upstream projects (like u-boot) and that they don't
      require too much extra maintenance. Having already a cleaned up
      u-boot to base the PKGBUILDs on could help factorizing the code and
      sharing the work on the u-boot code review process.
    - Other FSDG compliant distributions already have u-boot (like Guix)
      or might be interested in shipping u-boot (like Replicant), so it
      would be a good idea not to duplicate the work again and again.
    
    In the future the code that removes the nonfree software and the
    problematic documentation should be moved in a reusable script (that
    could still be run in mksource), possibly in its own package, in order
    to be reusable accross different distributions and u-boot PKGBUILDs.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>