Enhance target handling in Bootloader to support multiple targets

[gemenerik]

Support setting multiple targets per binary in manifest.json

For example;

        "color-led.bin": {
            "platform": "deck",
            "target": ["bcColorLEDBottom:color", "bcColorLEDTop:color"],
            "type": "fw",
            "release": "{{ color-led-firmware-version }}",
            "repository": "color-led-deck-firmware"
        }

Supports single target (string, ensures backwards compatibility), or multiple (list)

[Copilot]

Pull Request Overview

This PR enhances the bootloader's manifest parsing to support flashing a single binary to multiple targets. Previously, each binary in manifest.json could only specify one target (as a string). Now it supports both a single target (string) and multiple targets (list), maintaining backwards compatibility.

Key Changes:

• Added logic to handle target field in manifest.json as either a string or list
• Loop through all targets and create separate FlashArtifact instances for each
• Moved legacy nRF51 requirement logic inside the target loop

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[ataffanel]

Great concise efficient change!