Add libvpx and enable gst1-mod-vpx

I'm not sure if this should be divided into two PR. The objective of this PR is to enable gst1-mod-vpx.
However, as libvpx was missing, it was also added as a new package.

I added myself as libvpx maintainer but probably @thess might be a better maintainer.

This PR was compile tested with target ar71xx and runtime tested with x86 (only VP8 enc).