cmake: dtc: Add ability to check of dtc flag is valid
cmake: dtc: Add ability to check of dtc flag is valid

Add a function that let's us check if a given dtc flag can be used with
the version of dtc we have.  This lets us utilize specific warning flags
that might not exist in older dtc versions and still have things work
properly.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

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