v3/ec: update to build with ToT

Add a kconfig that allows building with tip-of-tree Zephyr and
Pigweed. Update documentation to show how to use Pigweed's bootstrap
to set up the build environment.

BUG=b:271448278, b:271984270
TEST=follow build instructions to get latest Zephyr and Pigweed
code and build an EC binary. Flash to a device and do some ad-hoc
tests to see that the EC works.

Change-Id: I08d984eac3521b2e7e673b927e61265a396e94b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/chameleon/+/4366422
Commit-Queue: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Yuval Peress <peress@google.com>
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Alexandru Stan <amstan@chromium.org>
2 files changed