stand/fdt: Fix loading of multiple fdt_overlays

fdt_load_dtb_overlays was written to unload previous overlay when a new
valid one is come across. fdt_apply_overlays further down is written to
iterate over all .dtbo's currently loaded and apply them one-by-one. Correct
fdt_load_dtb_overlays to stop dropping valid overlays that were previously
loaded and match expectations.

Reviewed by: gonzo, imp
Differential Revision: https://reviews.freebsd.org/D13659