sdl2_image: new package

sdl2_image: new package

Signed-off-by: Peter Thompson <peter.macleod.thompson@gmail.com>
[Thomas:
 - remove all sub-options to select the various image formats. Between
   no formats enabled and all formats enabled, the size difference of
   the library is ~30 KB, so it really isn't worth having all those
   sub-options:
   -rwxr-xr-x 1 thomas thomas  22444 juil. 15 15:51 libSDL2_image-2.0.so.0.0.1
   -rwxr-xr-x 1 thomas thomas  59216 juil. 15 15:52 libSDL2_image-2.0.so.0.0.1
   consequently, we simply enable unconditionally all image formats
   that don't have any dependency, and use "automatic" dependencies
   for the 4 formats that require an external library
 - remove the host package variant, as it isn't used anywhere.
 - remove --with-sdl2-prefix and --with-sdl2-exec-prefix, and instead
   add a dependency on host-pkgconf so that pkg-config is used
 - remove --disable-static, this is handled by the autotools-package
   infrastructure already
 - point <pkg>_LICENSE_FILES to an existing file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>