[feature] Add board_name field to Device model for DTB board identification

Is your feature request related to a problem? Please describe.

NOTE : This is a follow-up issue related to the GSoC 2026 project: Automatic Extraction of OpenWrt Firmware Image Metadata

The firmware upgrader module currently pairs firmware images to devices only via Device.model == FirmwareImage.board (exact match).

Multi-board OpenWrt images (one image supporting several hardware variants, distinguished only by DTB compatible strings) can't have a single board value, so they always require manual confirmation. Metadata extraction already populates each image's compatible list (openwisp-firmware-upgrader#437), but Device has no comparable field to match against.

Describe the solution you'd like

Add board_name field to the Device model to store the device's DTB board identifier (e.g. tplink,archer-c7-v2):

• CharField, blank/nullable, indexed if firmware-upgrader will filter on it
• Auto-populated from registration data, not user-editable (same treatment as os/system)
• Exposed in the REST API device serializer
• Migration included

This issue only adds the field. Populating it is a separate issue in openwisp-config

Describe alternatives you've considered

Keep pairing limited to board only, leaving multi-board images permanently dependent on manual confirmation. Works, but means a human has to intervene even when the device's hardware is unambiguous from its DTB identifier.

Additional context

Device.model is already populated the same way: read from the device during registration and stored automatically. board_name follows the identical mechanism, just carrying the DTB-format identifier instead of the human-readable one.

Depends on openwisp-config#282
Depends on openwisp-firmware-upgrader#493