cmake: add new search locations for shield overlay and conf files

* Add support to search for a board specific shield in
  <SHIELD DIR>/boards/<SHIELD>/<BOARD>.{overlay,conf}.  So we can
  support something like:

  x_nucleo_iks01a3/boards/x_nucleo_iks01a3_shub/stm32mp157c_dk2.overlay
  x_nucleo_iks01a3/boards/x_nucleo_iks01a3_shub/stm32mp157c_dk2.conf

Fixes #19078

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>