This make the process of applying overlays more reliable by:
1. Ignoring dtb files that are not really device trees. [^1]
2. Adding a `filter` option (per-overlay, there already is a global one)
to limit the files to which the overlay applies. This is useful
in cases where the `compatible` string is ambiguous and multiple
unrelated files match.
Previously the script would fail in both cases.
[^1]: For example, there is dtbs/overlays/overlay_map.dtb in the
Raspberry Pi 1 kernel.
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