Change for updating externel/dtc from upstream

The dtc merged the overlay patch, but changed the DTS header format
from:

  /dts-v1/ /plugin/;

to:

  /dts-v1/;
  /plugin/;

(See https://github.com/dgibson/dtc/commit/1ef86ad2c24f67567b1021ca5cb84bea82749f88)

Change all test cases in libufdt.

Bug: 35151782
Test: run test scripts (see libufdt/tests/README)
Change-Id: If13d04fff18bf449eb92ab54842117fe90d8ba88