cmake: Fixup shield overlay and conf file search locations

* Fix dir name when shield name doesn't match the dir name.  So for
  shield 'x_nucleo_iks01a3_shub' we will look in now for
  x_nucleo_iks01a3/boards/<BOARD>.{overlay,conf} instead of
  x_nucleo_iks01a3_shub/boards/<BOARD>.{overlay,conf}

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