bitbake-layers: list overlayed classes in show-overlayed

bitbake-layers: list overlayed classes in show-overlayed

Classes (.bbclass files) can be overlayed in a layer although they are
currently located by BitBake in a different way (via BBPATH instead of
using layer priority) and thus it is useful to be able to see when this
is in effect and which layer's class is actually being used.

(Bitbake rev: f6493e4bad005a82580380d800ebf4c438292f5b)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>