conf.py: clean up exit if ZEPHYR_BASE is unset
conf.py: clean up exit if ZEPHYR_BASE is unset

Suggested by Ulf Magnusson.

Signed-off-by: Marti Bolivar <fd5da480b758a78208111e6ae5c6806e1f887137@opensourcefoundries.com>

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