[yocto] Using a poky recipe's files in a custom recipe

[colin.helliwell at ln-systems.com]

The current Poky I'm using (krogoth) has a recipe for dropbear, but I want
to upgrade to use the latest version of dropbear.
For source control reasons, I don't want to modify what's in
meta/recipes-core/dropbear and therefore want to have my own recipe in my
own layer. Yet for cleanliness I'd also prefer not to have to replicate all
its .inc file etc in my own layer.
Is there a clean way to be able to include meta's files within my own
recipe? Is it a case of some sort of FILESEXTRAPATHS entry, and what would
be the best way to cross reference to the poky layer?