Add west.yml to make manifest-module.

Extending mcuboot to be a Zephyr manifest-module enables
mcuboot integration consistent with Zephyr's example-application.

Verified by:

```
west init https://github.com/$my_account/mcuboot

 \
     --mr $my_branch ~/$my_wksp && \
cd ~/$my_wksp && \
west update && \
source zephyr/zephyr_env.sh && \
west build -p always -b nrf52840dk_nrf52840 mcuboot/boot/zephyr
```

Fixes #1496

Signed-off-by: Gregory Shue <gregory.shue@legrand.com>