sdl: use <pkg>_CONFIG_SCRIPTS for sdl-config

Use the <pkg>_CONFIG_SCRIPTS mechanism to handle the sdl-config
script, so that it properly gets removed from the target. However, we
can't completely remove the manual fixup, because it also takes care
of removing the -Wl,-rpath option, which the general
<pkg>_CONFIG_SCRIPTS doesn't do.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>