Handle overlaying without node __symbols__

In new version of dtc, it does not always generate __symbols__ node:

https://github.com/dgibson/dtc/commit/e3c769aa9c16e7b94051c5eaae6635e79ac19385

Revises libufdt to handle main DT without node __symbols__.

Bug: 35230516
Test: tests/run_tests.sh
Change-Id: Ie086abc93ed4d45e4fe28c1d8c48fd37873256c1