libfdt: Enhanced and published fdt_next_tag()

Enhanced the formerly private function _fdt_next_tag() to allow stepping
  through the tree, used to produce a human-readable dump, and made
  it part of the published interface.
Also added some comments.
diff --git a/libfdt/fdt_wip.c b/libfdt/fdt_wip.c
index 0db7d25..261b9b0 100644
--- a/libfdt/fdt_wip.c
+++ b/libfdt/fdt_wip.c
@@ -68,12 +68,12 @@
 	uint32_t tag;
 	int offset, nextoffset;
 
-	tag = _fdt_next_tag(fdt, nodeoffset, &nextoffset);
+	tag = fdt_next_tag(fdt, nodeoffset, &nextoffset, NULL);
 	if (tag != FDT_BEGIN_NODE)
 		return -FDT_ERR_BADOFFSET;
 	do {
 		offset = nextoffset;
-		tag = _fdt_next_tag(fdt, offset, &nextoffset);
+		tag = fdt_next_tag(fdt, offset, &nextoffset, NULL);
 
 		switch (tag) {
 		case FDT_END: