The `(..)/raspberrypi-kernel/changelog.Debian.gz` file is not guaranteed
to be present in the built `ROOTFS_DIR`, for example when building very
minimal images without package documentation. In these cases, the `firmware`
variable will be left empty and the subsequent calls to `curl` will return
large 404 HTML content from GitHub.
Instead, simply check if the changelog file exists before using it.
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