libfdt: Rename and publish _fdt_next_tag()

Although it's a low-level function that shouldn't normally be needed,
there are circumstances where it's useful for users of libfdt to use
the _fdt_next_tag() function.  Therefore, this patch renames it to
fdt_next_tag() and publishes it in libfdt.h.

In addition, this patch adds a new testcase using fdt_next_tag(),
dtbs_equal_ordered.  This testcase tests for structural equality of
two dtbs, including the order of properties and subnodes, but ignoring
NOP tags, the order of the dtb sections and the layout of strings in
the strings block.  This will be useful for testing other dtc
functionality in the future.

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

tests/run_tests.sh

@@ -104,6 +104,7 @@ dtc_tests () {
     run_test dtc.sh -I dts -O dtb -o dtc_tree1.test.dtb test_tree1.dts
     tree1_tests dtc_tree1.test.dtb
     tree1_tests_rw dtc_tree1.test.dtb
+    run_test dtbs_equal_ordered dtc_tree1.test.dtb test_tree1.dtb
 
     run_test dtc.sh -I dts -O dtb -o dtc_escapes.test.dtb escapes.dts
     run_test string_escapes dtc_escapes.test.dtb