cmake: Fix CONFIG_CUSTOM_LINKER_SCRIPT
cmake: Fix CONFIG_CUSTOM_LINKER_SCRIPT

Specifying a custom linker script is supposed to be supported through
Kconfig, but evidently this has been unused and 100% broken at least
since the CMake migration.

This commit fixes
https://github.com/zephyrproject-rtos/zephyr/issues/6983

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

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