Revert "lava: Drop spurious tool setting"

This reverts commit 0f2e8a1a.

On commit 0f2e8a1a a clean up was introduced to remove validation warnings. Unfortunately the tool prompts is key is used by lava to detect when the deploy action has finished when the tool is different from dd as described in:

https://github.com/Linaro/lava/blob/c38d449719d7e4dca3d07e6b5e9bb63d5a5579a9/lava_dispatcher/actions/deploy/removable.py#L151

Revert the previous changes to allow LAVA to detect that the deploy action has finished.