of: overlay: expand check of whether overlay changeset can be removed

The test of whether it is safe to remove an overlay changeset
looked at whether any node in the overlay changeset was in a
subtree rooted at any more recently applied overlay changeset
node.

The test failed to determine whether any node in the overlay
changeset was the root of a subtree that contained a more
recently applied overlay changeset node.  Add this additional
check to the test.

The test is still lacking any check for any phandle dependencies.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>