build: update instructions to use Zephyr's west tool

To make this more familiar for Zephyr's users, git submodules were
replaced with west.

Change-Id: Ia10bb01318d0c43ea730698548f7bce6e70fd80c
Signed-off-by: Yuval Peress <peress@google.com>
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/120116
Reviewed-by: Wyatt Hepler <hepler@google.com>