SDL 2.0 has been released more than 7 years ago and should be available
everywhere. Replace barebox's SDL 1.2 with SDL 2.0 instead:

 - better maintained

 - At least on Debian, you can't install x86_64 and i386 libsdl-dev at
   the same time, which makes a quick use of CONFIG_SANDBOX_LINUX_I386
   harder as the user needs to reinstall libraries. With SDL 2.0, it
   works

 - SDL 2.0 has easier audio API, which will be used in a later commit

 - Wayland support for sandbox video driver

Port to SDL 2.0 and as we touch everything anyway, rename the
sdl_ functions to start with sdl_video_ to differentiate from upcoming
sdl_sound_.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>