libfdt: Make fdt_check_header() more thorough

Currently fdt_check_header() performs only some rudimentary checks, which
is not really what the name suggests.  This strengthens fdt_check_header()
to check as much about the blob as is possible from the header alone:  as
well as checking the magic number and version, it checks that the total
size is sane, and that all the sub-blocks within the blob lie within the
total size.

 * This broadens the meaning of FDT_ERR_TRUNCATED to cover all sorts of
   improperly terminated blocks as well as just a structure block without
   FDT_END.

 * This makes fdt_check_header() only succeed on "complete" blobs, not
   in-progress sequential write blobs.  The only reason this didn't fail
   before was that this function used to be called by many RO functions
   which are supposed to also work on incomplete SW blobs.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Simon Glass <sjg@chromium.org>