contrib: vpx: remove strip step and fix debug symbols
Without CONFIG_DEBUG set, libvpx is calling `$(STRIP) --strip-debug` on the final library and removes the debug symbols from it. With the variable set, it becomes a simple copy operation. In addition, debug symbols should be enabled for all builds since we strip in the end, so as to have the debug symbols in a separate file when doing releases. In case we build with optimization, remove the assertions.