gst-plugins-bad: unbreak vp8 support

gst-plugins-bad: unbreak vp8 support

Fixes
http://autobuild.buildroot.net/results/84e/84e7a2df4a8de2ca1ce075a8f5f32a94b7f45ce8/
http://autobuild.buildroot.net/results/db8/db8309c8cc6d4fb8a28d17c3b1ed71c49277d212/
http://autobuild.buildroot.net/results/5e4/5e424a25ee6796207e7a85ecbc80ac4428b0522e/
http://autobuild.buildroot.net/results/3b0/3b0e0356549d40edbe1c9a86214437e0f81fb3a3/

And many more.

gst-plugins-bad didn't have any explicit libvpx handling, so it would be
built with/without support for it depending on build order.

More importantly, the recent libvpx bump dropped the legacy (prefix-less)
symbols from the header file and gst-plugins-bad's handling of the non
legacy symbols was buggy.

Fix it by adding an explicit vp8 suboption and add a patch to fix (drop) the
legacy symbol handling.

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