Change libffmpeg-full to, by default, use the LGPL license. This
allows libffmpeg-full to gain support for libfdk-aac.
When libx264 is selected, this changes to GPL, and libfdk-aac
support is lost. Libx264 support is prioritized when both are
selected, which maintains the status quo of what happens now.
Signed-off-by: Ian Leonard <antonlacon@gmail.com>