sdl2: explicitly disable raspberry pi video backend

Fixes:
http://autobuild.buildroot.net/results/d59/d5992dcc9a49ee77afaebdcc9448ac1868fa7de1/
http://autobuild.buildroot.net/results/e89/e894f21ce1983ee3bd8d65a8e59e1adab9a62707/



The configure script automatically enables support for the raspberry pi
video backend if it detects the rpi-userland package.  Unfortunately it
hardcodes a number of include/linker paths unsuitable for cross compilation,
breaking the build:

    if test x$enable_video = xyes -a x$enable_video_rpi = xyes; then
..
     RPI_CFLAGS="-I/opt/vc/include -I/opt/vc/include/interface/vcos/pthreads -I/opt/vc/include/interface/vmcs_host/linux"
     RPI_LDFLAGS="-L/opt/vc/lib -lbcm_host"
    fi

So explicitly disable it until the configure script is fixed.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>