scripts: zephyr_module: hack to match west 0.7 output

The west command line output is not stable and has changed for 0.7.
Match it in zephyr_module.py's check for whether we are in a
workspace.

The real fix is to start using 'west topdir' whenever the west version
is at least 0.7.

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