cmake: Suppress the stderr from 'git describe' on successfull runs
cmake: Suppress the stderr from 'git describe' on successfull runs

A confusing warning has popped up since 3.13. It can be observed as
so:

Zephyr version: 1.13.99
Parsing Kconfig tree in /home/sebo/zephyr/Kconfig
Merging /home/sebo/zephyr/samples/hello_world/prj.conf
warning: tag 'zephyr-v1.13.0' is really 'v1.13.0' here
-- Generating zephyr/include/generated/generated_dts_board.h

This warning is more confusing than useful so we suppress stderr from
'git describe' when the command runs successfully.

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

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