BBHandler/cooker: Implement recipe and global classes
    
    We have some confusion for users since some classes are meant to work
    in the configuration space (or "globally") and some are meant to be
    selected by recipes individually.
    
    The cleanest way I could find to clarify this is to create "classes-global"
    and "classes-recipe" directories which contain the approproate classes and
    have bitbake switch scope between them at the appropriate point during
    parsing. The existing "classes" directory is always searched as a fallback.
    
    Once a class is moved to a specific directory, it will no longer be found
    in the incorrect context. A good example from OE is that
    
    INHERIT += "testimage"
    
    will no longer work but
    
    IMAGE_CLASSES += "testimage"
    
    will, which makes the global scope cleaner by only including it where it
    is useful and intended to be used (images).
    
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>