libfdt: overlay: Report a bad overlay for mismatching local fixups

The __local_fixups__ node as a structure that mimics the structure of the
main overlay part.

This means that if we have a child node somewhere in the local fixups
sub-tree and if that node is not present in the main tree, the overlay is
poorly formatted, and we should report it as such.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>