Currently fdt_next_node() will find the next node in the blob
regardless of whether it is above, below or at the same level in the
tree as the starting node - the depth parameter is updated to indicate
which is the case.  When a depth parameter is supplied, this patch
makes it instead terminate immediately when it finds the END_NODE tag
for a node at depth 0.  In this case it returns the offset immediately
past the END_NODE tag.

This has a couple of advantages.  First, this slightly simplifies
fdt_subnode_offset(), which no longer needs to explicitly check that
fdt_next_node()'s iteration hasn't left the starting node.  Second,
this allows fdt_next_node() to be used to implement
_fdt_node_end_offset() considerably simplifying the latter function.

The other users of fdt_next_node() either don't need to iterate out of
the starting node, or don't pass a depth parameter at all.  Any
callers that really need to iterate out of the starting node, but keep
tracking depth can do so by biasing the initial depth value.

This is a semantic change, but I think it's very unlikely to break any
existing library users.

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