Make OTA the default K2 build path

[lindestad]

This updates the K2 build and flashing flow for #28.

The build wrappers now default to the H7 OTA sysbuild, keep --ota as an explicit no-op, and require --no-ota for a plain development build. F7 flags now fail clearly because F7 support is being sunset.

The OTA helpers now discover the signed image under build-h755-ota/*/zephyr/zephyr.signed.bin, so sibling worktrees and renamed checkouts do not break the default upload path. The README, Ethernet OTA guide, release matrix, and stale comments now document H7 and OTA as the supported path, with USB west flash reserved for first-time provisioning or recovery.

Validated with:

• ./build.sh --help
• ./build.sh --f7 exits with the sunset message
• pwsh -NoProfile -File ./build.ps1 --help
• pwsh -NoProfile -File ./build.ps1 --f7 exits with the sunset message
• bash -n build.sh tools/k2-ota.sh
• PowerShell parser check for build.ps1 and tools/k2-ota.ps1
• ./build.sh default OTA build succeeds and produces build-h755-ota/K2-Zephyr-issue28-build-docs/zephyr/zephyr.signed.bin

[Copilot]

Pull request overview

Makes the H7 MCUboot/OTA sysbuild the default for build.sh/build.ps1, sunsets F7 flags with a clear error, teaches the OTA helpers to discover the signed image via a build-h755-ota/*/zephyr/zephyr.signed.bin glob, and updates README/ETHERNET_OTA/comments/CMake messaging to frame Ethernet OTA as the normal update path and USB west flash as provisioning/recovery only.

Changes:

• Default build is now H7 OTA; --ota is a no-op, --no-ota opts into plain builds, --f7 errors out.
• OTA helpers (tools/k2-ota.sh/.ps1) auto-locate the signed image under build-h755-ota/*/zephyr/zephyr.signed.bin, removing the hardcoded K2-Zephyr path.
• Docs, comments, and a new CMake status message reorient users toward Ethernet OTA, reserving west flash for first-time/recovery use.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
build.sh	Default to OTA, add --no-ota, reject --f7, robust workspace/venv paths, new flash guidance output.
build.ps1	PowerShell mirror of the new defaults, flags, venv lookup, and flash guidance.
tools/k2-ota.sh	Compute project dir; glob-discover default signed image with clear multi/zero match errors.
tools/k2-ota.ps1	Mirror discovery in PowerShell; default $Image from $env:IMAGE.
README.md	Document H7-only support, OTA-default build, USB flash as provisioning/recovery.
ETHERNET_OTA.md	Add provisioning section pointing to ./build.sh and one-time USB flash.
CMakeLists.txt	Emit a STATUS hint clarifying when to use west flash vs OTA helpers.
prj.conf	Update Ethernet comment to target H755 only.
src/main.c	Header comment now lists only the H755ZI-Q target.
.github/workflows/main.yml	Drop F7 from the release matrix.

Comments suppressed due to low confidence (1)

ETHERNET_OTA.md:158

• These lines still hardcode build-h755-ota/K2-Zephyr/zephyr/zephyr.signed.bin, which is inconsistent with the helper change in this PR that discovers the signed image under build-h755-ota/*/zephyr/zephyr.signed.bin. For users whose checkout directory is not literally named K2-Zephyr (a sibling worktree, a fork rename, etc.), both the prose ("By default, the helper finds the signed image at ...") and the explicit mcumgr ... image upload ... example will be wrong. Consider documenting the wildcard path or explaining that the middle component is the source-directory name.

By default, the helper finds the signed image at
`build-h755-ota/K2-Zephyr/zephyr/zephyr.signed.bin`. To upload a different
signed image:

```bash
./tools/k2-ota.sh path/to/zephyr.signed.bin

On Windows:

.\tools\k2-ota.ps1

Manual fallback:

mcumgr --conntype udp '--connstring=[10.77.0.2]:1337' image upload build-h755-ota/K2-Zephyr/zephyr/zephyr.signed.bin

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