Add persistent storage partition for ML models and data

[mihai-chiorean]

Description

We need a dedicated partition for storing persistent data that survives Mender OTA updates. This partition will be used for ML models, application data, and other persistent storage needs.

Note: Mender OTA is not available yet. However, when implementing a solution, consider if an A/B OTA update pattern would conflict with the implementation.

Requirements

• Create a separate partition for persistent storage
• Partition must survive Mender A/B updates
• Should be mounted at a standard location (e.g., /data or /persistent)
• Must be integrated with Mender configuration

Technical Considerations

• Modify WIC/WKS file to include data partition
• Update Mender configuration to exclude this partition from updates
• Ensure partition is formatted and mounted automatically at boot
• Consider partition sizing strategy (fixed size vs. expandable)

Acceptance Criteria

• Data partition created during image build
• Partition automatically mounted at boot
• Documentation updated with partition layout