cmake: Fixup shield overlay and conf file search locations
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 <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

nashif/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr