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

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.

Reviewed by: gonzo
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13968