electric-eel: add initial project and docs

Create the initial Zephyr project to run on the STM32H743 evaluation
board.
Add some documentation about how to build and flash on the target.

BUG=None
TEST=Build and flash per the docs. Open the serial console and read
Zappy's advice.

Change-Id: I59536e094d6ce6aebb9cf44dc8d69aadbf0a5992
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/electric-eel/+/3733522
Reviewed-by: Alexandru Stan <amstan@chromium.org>
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
8 files changed