MFC (conceptually) r327298, r327299: Fix overlay application behavior

This is a direct commit to stable/11 due to restructuring of sys/boot =>
stand in -HEAD. The diff remains the same and it simply applied to the
previous location.

MFC r327298: 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.

MFC r327299: stand/fdt: Avoid bailout when dtbo has no fixups

In the case of a simple dtbo where fragment uses target-path and the overlay
contains no references, /__fixups__ will not be included by either our dtc
or dtc from ports, but the file still has valid fragments to be applied.

Additional testing found that /__symbols__ might also be omitted if it's
empty, which is not necessarily an error.