configure: add option --enable-vp8-role-name, default off (chromium)

1. when --enable-vp8-role-name is not set (default)
   vp8 decoder uses "video_decoder.vpx" as role name, it matches chromium
2. when --enable-vp8-role-name is set,
   vp8 decoder uses "video_decoder.vp8" as role name, it matches gst-omx