dtoc: Support packing the device tree

After any node/property deletion the device tree can be packed to remove
spare space. Add a way to perform this operation.

Note that for fdt_fallback, fdtput automatically packs the device tree after
deletion, so no action is required here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/libfdt/libfdt.swig b/lib/libfdt/libfdt.swig
index ce516fd..0cb7977 100644
--- a/lib/libfdt/libfdt.swig
+++ b/lib/libfdt/libfdt.swig
@@ -107,3 +107,4 @@
 int fdt_delprop(void *fdt, int nodeoffset, const char *name);
 
 const char *fdt_strerror(int errval);
+int fdt_pack(void *fdt);