Re-use already built libdragon tools

[anacierdem]

Since libdragon tools will be compiled as part of the local libdragon copy, there must be a way to exec them without having to reinstall them every time the container is lost. Possibly launching them from the build directory or something like that.

This is possible using docker volumes but might require a mechanism to detect if the tools were changed or not. Ideally we want this to be handles by libdragon Makefiles but they are not ready yet. Using build.sh will re-build them regardless.