A list can contain the same value twice, therefore only append to the
appendlist for a recipe if the append file is not already in the list.
(Bitbake rev: d11f9dd33cdcc97a4a937e8bf7e97558d813cadd)
Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>