cmake: Fix support for "ninja flash"

By default west looks at the parents directory to find Zephyr base
directory. However, in the case the build directory is not a subdirectory
of zephyr base and the user run "ninja flash" directly from the build
directory, west fails to detect Zephyr base.

So, this patch explicitly add ZEPHYR_BASE in the environment.

Implementation notes:
  * ZEPHYR_BASE is already set for ninja menuconfig in
    cmake/modules/kconfig.cmake:201
  * ZEPHYR_BASE seems better than --zephyr-base since the environment in
    inherited
  * BTW, --zephyr-base seems to not have any impact and seems broken

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>