xbmc: new package

XBMC is an award-winning free and open source (GPL) software media player and
entertainment hub for digital media.
This package was originally found at : https://github.com/huceke/buildroot-rbp
By gimli <ebsi4711@gmail.com>

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: gimli <ebsi4711@gmail.com>
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Changes v3 -> v4:
  - Rework init script
  - Add systemd startup service
Changes v2 -> v3:
  - Fix indent
  - Change librtmp to rtmpdump
Changes v1 -> v2:
  - Fix header (Yann E. Morin)
  - Fix comment on toolchain option (Peter Korsgaard)