build: T3664: use explicit defaults argument in the dict merging function (backport #606)

[mergify]

Use explicit defaults argument in the dict merging function to make it clear what is merged into what,
and correct some instances of incorrect argument order along the way.

Change Summary

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Component(s) name

Build scripts.

How to test

$ cat data/build-flavors/raw-test.toml

image_format = "raw"

packages = ["sl"]

[boot_settings]
  console_type = "ttyS"
  console_speed = 9600

[[includes_chroot]]
  path = "etc/test"
  data = "TEST!"

$ sudo ./build-vyos-image --debug --dry-run raw
I: Checking if packages required for VyOS image build are installed
I: using build flavors directory data/build-flavors
D: Effective build config:

{
    "build_type": "development",
    "architecture": "amd64",
    "debian_distribution": "bookworm",
    "debian_mirror": "http://deb.debian.org/debian",
    "debian_security_mirror": "http://deb.debian.org/debian-security",
    "debian_archive_areas": "main contrib non-free non-free-firmware",
    "vyos_mirror": "https://rolling-packages.vyos.net/current",
    "vyos_branch": "current",
    "release_train": "current",
    "kernel_version": "6.6.30",
    "bootloaders": "syslinux,grub-efi",
    "squashfs_compression_type": "xz -Xbcj x86 -b 256k -always-use-fragments -no-recovery",
    "website_url": "https://vyos.io",
    "support_url": "https://support.vyos.io",
    "bugtracker_url": "https://vyos.dev",
    "documentation_url": "https://docs.vyos.io/en/latest",
    "project_news_url": "https://blog.vyos.io",
    "packages": [
        "sl",
        "grub2",
        "grub-pc",
        "vyos-linux-firmware",
        "vyos-intel-qat",
        "vyos-intel-ixgbe",
        "vyos-intel-ixgbevf",
        "openvpn-dco",
        "telegraf",
        "gdb",
        "strace",
        "apt-rdepends",
        "tshark",
        "vim",
        "vyos-1x-smoketest"
    ],
    "additional_repositories": [
        "deb [arch=amd64] https://repo.saltproject.io/py3/debian/11/amd64/3005 bullseye main"
    ],
    "kernel_flavor": "amd64-vyos",
    "image_format": [
        "raw"
    ],
    "boot_settings": {
        "timeout": "5",
        "console_type": "ttyS",
        "console_num": "0",
        "console_speed": 9600,
        "bootmode": "normal"
    },
    "includes_chroot": [
        {
            "path": "etc/test",
            "data": "TEST!"
        }
    ],
    "build_by": "root@aa73acc27c0d",
    "pbuilder_debian_mirror": "http://deb.debian.org/debian",
    "version": null,
    "build_comment": "",
    "debug": true,
    "dry_run": true,
    "custom_apt_entry": [],
    "custom_apt_key": [],
    "custom_package": [],
    "reuse_iso": null,
    "disk_size": 10,
    "build_flavor": "raw",
    "build_dir": "build",
    "pbuilder_config": "build/pbuilderrc"
}
W: Could not build a version string specific to git branch, falling back to default: 'T3664-dict-merge-fixes'
I: Cleaning the build workspace
...

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #606 done by [Mergify](https://mergify.com).