RDKDEV-1414 Add RDK Firmware Update Manager Documentation

[gourivarma3]

Added comprehensive documentation for the RDK Firmware Update Manager, detailing its architecture, features, and operational workflows.

[gourivarma3]

I have read the CLA Document and I hereby sign the CLA

[satya200]

Please rebase the branch and take a look into openspec folder. you will find all the documents. If anything miss please update in the same folder

[Copilot]

Pull request overview

Adds a new, comprehensive docs/README.md describing the RDK Firmware Update Manager daemon and its client library, including architecture diagrams, module breakdown, IPC flows, and configuration details.

Changes:

• Introduces a detailed component overview (architecture, threading model, state flow, and call flows) with Mermaid diagrams.
• Documents internal modules and integration points (D-Bus, IARM, XConf/CDN, flash HAL).
• Adds configuration and runtime persistence details (RFC parameters, cache/status files, systemd dependencies).

Comments suppressed due to low confidence (4)

docs/README.md:60

• This bullet says the service is registered on the system bus under org.rdkfwupdater.Interface, but the well-known bus name is org.rdkfwupdater.Service and the interface is org.rdkfwupdater.Interface (see src/dbus/rdkv_dbus_server.h). Consider documenting service name + object path + interface to avoid confusion.

- **D-Bus Service Interface**: A GDBus-based service is registered on the system bus under `org.rdkfwupdater.Interface`. It exposes methods (`RegisterProcess`, `UnregisterProcess`, `CheckForUpdate`, `DownloadFirmware`, `UpdateFirmware`) and emits signals (`CheckForUpdateComplete`, `DownloadProgress`, `UpdateProgress`) for asynchronous operation results.

docs/README.md:153

• The synchronization description is inaccurate/misleading: the XConf cache is protected via GLib G_LOCK(xconf_data_cache) (not a GMutex named g_xconf_data_cache), and mutuex_dwnl_state / app_mode_status are pthread_mutex_t (see src/rdkFwupdateMgr.c / src/rdkv_main.c). Also, GHashTable doesn’t provide “built-in reference counting” for entries; it’s just used as a container with destroy callbacks.

- **Synchronization**: `GMutex` protects the global XConf data cache (`g_xconf_data_cache`), the XConf in-progress status (`xconf_comm_status`), the download state (`mutuex_dwnl_state`), and the application mode state (`app_mode_status`). `GHashTable` with GLib's built-in reference counting is used for client registration tracking.

docs/README.md:204

• In this sequence diagram the daemon is shown registering org.rdkfwupdater.Interface, but the name owned on the bus is org.rdkfwupdater.Service (the interface is org.rdkfwupdater.Interface). Aligning the diagram with the actual D-Bus naming avoids confusion when tracing name ownership.

    Daemon->>DBus: Register org.rdkfwupdater.Interface
    DBus-->>Daemon: Name acquired

docs/README.md:258

• This call flow says org.rdkfwupdater.Interface is acquired, but the well-known name acquired is org.rdkfwupdater.Service (see BUS_NAME in src/dbus/rdkv_dbus_server.h). The interface is exposed under that service name.

    Daemon->>DBus: register_dbus_service()
    DBus-->>Daemon: org.rdkfwupdater.Interface acquired
    Daemon->>Daemon: g_main_loop_run()

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