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 <marti@opensourcefoundries.com>