MFC r328293: stand/fdt: Fallback to name + .dtbo if we fail to load name

[This is a direct commit to stable/11 due to path restructuring in -HEAD;
diffs have been applied to their old locations]

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 move this from a hard-coded ".dtbo" to a loader.conf(5)
configuration option.