Add SDL_sound package

Closes #477

Peter: minor tweaks:
 - sdl_snd -> sdl_sound
 - cleaned up help text
 - don't transform binary name
 - add option to keep playsound/playsound_simple - Off by default
 - fix uninstall when BR2_HAVE_DEVFILES isn't enabled

Signed-off-by: Evan Zelkowitz <evan.zelkowitz@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>