d.keys() was not returning items which had no base content but the
variable came into existance through overrides. We have to process
self.overridedata to find these other variables.
(Bitbake rev: 77fc57c610d7f65f7e328ad555f525b2f8bdf1d7)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>