image: Exif "Model" tags should store camera model, not ID

[njhollinghurst]

It seems to be more in the spirit of the Exif and DNG specs to record the camera's "model" string, rather than its ID (which is neither a model nor a unique serial number), and to prefix a "Make" onto UniqueCameraModel.

The Make ("Raspberry Pi") is now a #define so that it can be overridden by Makefiles, but I'm happy to rework this.

Before:

Make                            : Raspberry Pi
Camera Model Name               : /base/soc/i2c0mux/i2c@1/imx708@1a
Unique Camera Model             : /base/soc/i2c0mux/i2c@1/imx708@1a

After:

Make                            : Raspberry Pi
Camera Model Name               : imx708_wide
Unique Camera Model             : Raspberry Pi imx708_wide

Here are some possible reasons why we might not want this change:

• It changes behaviour that users may already be relying on
• In a multi-camera setup (e.g. CM4), the camera's ID is no longer recorded
• It might change again if we change the way we handle variants (cameras with the same sensor)
• It misleadingly suggests that Raspberry Pi is the manufacturer of the named sensors, rather than OmniVision or Sony
• Libcamera's "Model" property "is not guaranteed to be stable or have any other properties required to make it a good candidate to be used as a permanent identifier of a camera."