libfdt: Rename and publish _fdt_next_tag()

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> 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 at gibson.dropbear.id.au>

Applied.

Thanks,
jdl