Add a Kokoro release/signed build

- Adds config and script to build and then sign debs
- Add flash.sh to the build artifacts, and modify it to prefer artifacts
in the same folder as it (so a user can just run flash.sh from x20 or
whatever, and get that image flashed)
- release.sh invokes continuous.sh to build the world, then generates
and signs a deb repo, and exports it to the artifacts dir with the rest
of the build
- Companion Kokoro config CL: cl/204976989

Change-Id: I2dff45549d2c1e2d945ce2d6d12105442b161aeb
10 files changed