[yocto] Making recipes depend on specific layers

[Philip Balister]

I run into problems (typically with BSP layers) where I want the layer 
to build only against oe-core, but I also would like to have recipes 
that depend on other layers. Typically, a "complex" image that uses 
packages built from other layers.

Now, I could just make a shim layer that contains the recipes that 
depends on a larger set of layers, but this gets annoying.

What I'd like to do is provide a way for recipes to say they depend on 
other layers, but if that layer is not present, parsing should not fail. 
Obviously, attempting to build such a recipe would fail (with a suitable 
error message).

Richard showed me some Python I can use to do this, but I'd like to know 
if this seems useful to other people.

Obviously, switching layer sin and out can lead to madness, but I'd 
rather not try to protect the end user from every stupid thing they can 
do at the expense of making thing more complex from a shear number of 
layers view.

Philip