SDL_net: add new package SDL_net

This patch adds a new package SDL_net. SDL_net is a small, low-level,
cross-platform network library, that can be used with the Simple DirectMedia
Layer library (SDL).

http://www.libsdl.org/projects/SDL_net/



Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>