sysupgrade: read manifest, learn --channel/--build/--list-builds (1.0.50)

[widgetii]

Summary

PR-C of six. Teaches on-device sysupgrade to read the busybox-friendly manifest.flat published by #2112 at https://openipc.github.io/firmware/manifest.flat. No new rootfs dependency — parsing is pure busybox awk/grep.

New flags

Flag	Behaviour
--channel=<nightly|latest>	Resolve via @channel records in the manifest; pin to the rolling channel target.
--build=<nightly-YYYYMMDD-xxxxxxx>	Pin to a specific dated build (downgrade / bisect).
--list-builds[=N]	Print the last N (default 20) builds for this camera's ${soc}_${osr} platform; mark the running build with * using BUILD_ID from /etc/os-release (#2111).

scr_version bumped 1.0.49 → 1.0.50 and self_update()'s regex now fires on --channel/--build so users on a stale script get the manifest-aware version pulled before flashing.

Backward compatibility

download_firmware() resolves the manifest into a url before the legacy fw_printenv/releases/download/latest/ fallback, so unmodified invocations (-k / -r / --url / --archive) keep their exact prior behaviour. No flag day.

Local verification

Tested against a hand-crafted manifest.flat covering three dated builds across two platforms (hi3520dv200_lite, gk7205v200_lite):

=== resolve_channel nightly ===
nightly-20260520-b9d5883

=== resolve_url nightly-20260519-aabbccd hi3520dv200_lite nor ===
https://github.com/.../openipc.hi3520dv200-nor-lite.tgz

=== list builds for hi3520dv200_lite, running == nightly-20260519-aabbccd ===
    nightly-20260520-b9d5883
  * nightly-20260519-aabbccd
    nightly-20260518-1111111

Empty/unknown-id error paths die with explicit messages.

Test plan (on physical hardware after merge)

• ssh root@openipc-hi3520dv200.dlab.torturelabs.com sysupgrade --list-builds=5 returns the friendly empty-state message right now (no dated nightly-* releases exist yet).
• After tonight's first cron, same command lists ≥1 entry with * on the running build (gates BUILD_ID/manifest end-to-end).
• ssh root@openipc-gk7205v200.dlab.torturelabs.com sysupgrade --list-builds — same, on a board whose defconfig does NOT enable BR2_PACKAGE_JSONFILTER, proving zero JSON-parser dependency (per kaeru lab-gk7205v200-now-online).
• sysupgrade --channel=nightly flashes the rolling channel target.
• sysupgrade --build=<historic-id> flashes the named build.
• sysupgrade --url=… legacy path unchanged.

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