compatlayer/__init__.py: Allow add_layer to process recursive deps

compatlayer/__init__.py: Allow add_layer to process recursive deps

When processing a layer for dependencies, you have to process the layer
itself, it's dependencies, the dependencies dependencies and so forth until
all items have been processed.

i.e.:  LayerA requires LayerB requires LayerC requires layerD

The end result should be LayerB, LayerC and LayerD are all dependencies of
LayerA.

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>