checklayer: new function get_layer_dependencies()

checklayer: new function get_layer_dependencies()

Split add_layer_dependencies() into 2 parts. First search for layer
dependencies, and then add them to the config. That allows us to
call get_layer_dependencies() independently.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>