fdtoverlay: Ignore symbols in overlays which don't apply to the target tree

Symbols from overlays are merged into the target tree, and are required to
have the form:
	/fragment@XXX/__overlay__/...
If any symbols don't have this form, the overlay is rejected.

But there's not really anything wrong with an overlay having "local"
labels referring to a fragment node or some other metadata, that's not
expected to end up in a target tree.

So change our overlay application to simply ignore such symbols rather than
fail.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>