Remove hard-coded values for "bootloader" when creating the live-build,
replaced with the value of `bootloaders`.
Set the current value to the default value if not specified, via
`defaults.toml`.
Add the default for arm64 as `grub-efi`.
Currently, additional_repositories in the effective build_config are
ignored due to a bug wherein all values end up under
`args['custom_apt_entry']` rather than
`build_config['custom_apt_entry']`.
This change fixes that, ensuring that the content of
`build_config['additional_repositories]` is added to
`build_config['custom_apt_entry']` alongside the command-line arguments.
custom_apt_entry, custom_apt_key, and custom_package are expected to
be lists, but when a value is not provided they default to None. This
prevents lists provided in mix-in or flavor configurations from being
merged as expected.
https://docs.python.org/3/library/argparse.html#default
Rather than only looking at the default config, look at the merged
build_config.
Check wether additional_repositories is actually defined before trying
to reference it.
