gstreamer-omx: Improve variable expansion of ${S}

gstreamer-omx: Improve variable expansion of ${S}

The way ${S} is expanded into CFLAGS means the recipe is marked as depending
on the path within which its built. We can avoid this by avoiding the expansion
which then allows reuse of the recipe from sstate.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>