scripts: west: downgrade missing ZEPHYR_BASE to a warning
scripts: west: downgrade missing ZEPHYR_BASE to a warning

This is not strictly necessary for flashing and debugging, and is
causing issues in Linux environments where users run "make flash" as
root instead of installing udev rules.

Fixes: #7676

(Though we will need to revisit this when adding commands that run
CMake).

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

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