Validate required bootloader fields before dictionary access

[Copilot]

Addresses feedback from PR #5 review: accessing selected_bl["bootloader_file"], selected_bl["linker_script"], selected_bl["max_size"], and selected_bl["max_ram_size"] without validation raises KeyError on incomplete board configurations.

Changes

• Validation: Check for required fields (bootloader_file, linker_script, max_size, max_ram_size) before dictionary access
• Error handling: Exit with descriptive error message listing missing fields and bootloader name

# Before
if selected_bl:
    env.Replace(BOOTLOADERHEX=join(FRAMEWORK_DIR, "variants", 
                board.get("build.variant", ""), selected_bl["bootloader_file"]))

# After  
if selected_bl:
    required_fields = ["bootloader_file", "linker_script", "max_size", "max_ram_size"]
    missing_fields = [field for field in required_fields if field not in selected_bl]
    if missing_fields:
        sys.stderr.write("Error. Bootloader '%s' is missing required fields: %s\n" %
                        (selected_bl.get("name", "unknown"), ", ".join(missing_fields)))
        env.Exit(1)
    
    env.Replace(BOOTLOADERHEX=join(FRAMEWORK_DIR, "variants",
                board.get("build.variant", ""), selected_bl["bootloader_file"]))

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