cmake: Add support for out-of-tree shields
cmake: Add support for out-of-tree shields

Add support for out-of-tree shields. Out-of-tree shields can now be
stored in out-of-tree board directories.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,punitvara/zephyr,ldts/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,punitvara/zephyr,nashif/zephyr,nashif/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,explora26/zephyr,punitvara/zephyr