libre: uboot4extlinux-sunxi: lower maintenance costs
    
    - We can now do cross builds of u-boot:
      - We can now more easily work on the package as we don't need
        an ARM computer for that anymore.
      - It also enable people already using Parabola on x86 to more
        easily install, test, or repair u-boot for an ARM computer
        as users can simply install it on microSD cards without
        needing to use ARM chroots.
    - There is now an install script to install u-boot:
      - It makes things easier for users as it is more similar to
        grub-install than dd commands.
      - It also does many checks at the same time to avoid data loss.
    - The commands to split packages are now shared between all
      package_ functions. This lower maintenance.
    - How to add a new board has been made more clear.
    
    Comparison with other distributions (for consistency):
    - u-boot doesn't have anything to handle the installation and
      update of u-boot binaries if UEFI is not used.
    - Debian has an u-boot-install-sunxi script which handles way more
      cases than what I wrote (it even handles GPT and so on and even
      has a manual), but there seems to be no tools to update u-boot.
    - Openembedded doesn't seem to have anything special in
      the u-boot recipe in openembedded-core
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>