cmake: Invoke west topdir only with version >= 0.7.1
cmake: Invoke west topdir only with version >= 0.7.1

West version 0.7.0 introduced `west topdir` command.
Unfortunately version 0.7.0 would print the path in windows path style
when executed in Windows.

This commit ensure that `west topdir` is only used if west >= 0.7.1.

Signed-off-by: Torsten Rasmussen <a7c5b1d5a1f41fce6edfa3f7c2392ae0c59deafe@nordicsemi.no>

finikorg/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr