fdt: Add a function to remove unused strings from a device tree

Property names are stored in a string table. When a node property is
removed, the string table is not updated since other nodes may have a
property with the same name.

Thus it is possible for the string table to build up a number of unused
strings. Add a function to remove these. This works by building a new device
tree from the old one, adding strings one by one as needed.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/libfdt.h

@@ -1646,6 +1646,23 @@ int fdt_del_node(void *fdt, int nodeoffset);
 
 const char *fdt_strerror(int errval);
 
+/**
+ * fdt_remove_unused_strings() - Remove any unused strings from an FDT
+ *
+ * This creates a new device tree in @new with unused strings removed. The
+ * called can then use fdt_pack() to minimise the space consumed.
+ *
+ * @old:	Old device tree blog
+ * @new:	Place to put new device tree blob, which must be as large as
+ *		@old
+ * @return
+ *	0, on success
+ *	-FDT_ERR_BADOFFSET, corrupt device tree
+ *	-FDT_ERR_NOSPACE, out of space, which should not happen unless there
+ *		is something very wrong with the device tree input
+ */
+int fdt_remove_unused_strings(const void *old, void *new);
+
 struct fdt_region {
 	int offset;
 	int size;