cortex-m: add feature struct (unused) start to set up the hardware Change-Id: Ie12381efe614ec034c4dcdafddef15a29724d203 Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/maize/+/261714 Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com> Commit-Queue: Erik Gilling <konkers@google.com> Reviewed-by: Erik Gilling <konkers@google.com> Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>