Make sure the channel was not already added, before attempting to add.
[YOCTO #5890]
(From OE-Core rev: 0574f5390fb8958db5864f41a7bd8037ac046ac5)
Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>