gst1-plugins-ugly: Add option for building x264 plugin

As x264 is packaged, we can also build the gstreamer plugin using it.
This commit adds the option in menuconfig and make sure it is built if
selected.

[Peter: split fix, add GPL tag, update help text and commit description]
Signed-off-by: Vincent Dehors <vincent.dehors@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>