[yocto] Use recipes but not whole layer

[Oliver]

>>> In order to success in generate recipeA, I need to extend the boost to
>>> expand some configuration parameters(log).
>>> On the other side someone would like to use recipeB. He/She will
>>> unwillingly inherit the boost modifications.
>>>
>> 
>> The changes to boost would likely count as distro policy, so shouldn't be
>> in the same layer as a generic purpose recipe.

> What boost changes? They may be of interest to others and we could
> update the recipe in oe-core.

This particular change in boost is not relevant in fact, I just used it to explain the example. 

But if you like to know the bbappend file just contained the line:
    BOOST_LIBS += "log"