* Rename add_layer() to add_layers() so that add_layer_dependencies() can
re-use it.
* Avoid adding the layer if it is already present
[YOCTO #13148]
(From OE-Core rev: b9cc18d83f55ff48c3d6e60c56359f6736d5a06a)
(From OE-Core rev: 64310d9d18bb1751595c197e6955774ad5514844)
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>