libfdt: Improve sequential write state checking

When creating a tree with the sequential write functions, certain things
have to be done in a certain order.  You must create the memory reserve map
and only then can you create the actual tree structure.

The -FDT_ERR_BADSTATE return code is for if you try to do things out of
order.  However, we weren't checking that very thoroughly, so it was
possible to generate a corrupted blob if, for example, you started calling
fdt_begin_node() etc. before calling fdt_finish_reservemap().

This makes the state checking more thorough disallow that.

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>