The script expects a `finalize` directory in the root of romfs, which
is where all the files are.
3dstool is simple-minded; when specified a folder as the romfs target,
that folder becomes the root of romfs. This causes the built romfs to
have no `finalize` directory.
Move `finalize` to `romfs/finalize` and use the `romfs` directory for
3dstool.
Add a temporary variable that can be replaced externally, via `sed` or
other mechanism.
Let our GitHub Actions workflow add the sha256sum of the built romfs.
