sdl: fix incorrect SDL_CONF_OPTS assignment

The option --enable-video-directfb=no was being assigned to
SDL_CONF_OPTS instead of being appended to it when
BR2_PACKAGE_SDL_DIRECTFB was disabled. That would clobber any options
previously added to SDL_CONF_OPTS (namely --enable-video-fbcon=yes|no).

Signed-off-by: Rodrigo Rebello <rprebello@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>