scripts: fix west sign when ZEPHYR_BASE is unset

Due to cleanups in west targeted at getting rid of zephyr-specific
code, extension commands can no longer rely on ZEPHYR_BASE being set
in the calling environment at import time (it's still set at run()
time for now, though, to keep west build working).

Add a new helper to make dealing with this easier from west sign.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>