package/libtorrent-rasterbar: new package

libtorrent is a feature complete C++ bittorrent implementation
focusing on efficiency and scalability.

https://www.libtorrent.org/



Signed-off-by: Philipp Richter <richterphilipp.pops@gmail.com>
[Thomas: license is BSD-3c, not BSD-2c]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>