sdl2: Replace sdl with sdl2. Update to version 2.0.16

sdl2: Replace sdl with sdl2. Update to version 2.0.16

- Update from 1.2.15 (2013) to 2.0.16 (2021)
- Source file name changed from SDL to SDL2 so also deleted old sdl and created sdl2
   files for rootfile and lfs
- Changelog is too large to include here. Details can be found in the WhatsNew.txt file
   in the source tarball

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>