libfdt: Rename and publish _fdt_check_header()

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> It's potentially useful for users of libfdt to sanity check a device
> tree (or, rather, a blob of data which may or may not be a device
> tree) before processing it in more detail with libfdt.
> 
> This patch renames the libfdt internal function _fdt_check_header() to
> fdt_check_header() and makes it a published function, so it can now be
> used for this purpose.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

Thanks,
jdl