dts: Allow users to pass extra flags to DTC
dts: Allow users to pass extra flags to DTC

Allow application build scripts to pass extra flags to DTC, this could
for instance be done to enable/disable warnings.

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

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