This behavior also matches a Linux-ism by allowing fdt_overlays to specify names of overlays without an extension (e.g. fdt-overlays="sunxi-h3-h5-emac")
If we fail to load the file given by a name in fdt_overlays, try again with ".dtbo" appended to it. This still allows overlays to lack .dtbo extension if user prefers it and just adds a fallback cushion.
Future work could might this from a hard-coded ".dtbo" to loader.conf(5) configuration option.