libfdt: Add helper function to create a trivial, empty tree

The libfdt read/write functions are now usable enough that it's become a
moderately common pattern to use them to build and manipulate a device
tree from scratch.  For example, we do so ourself in our rw_tree1 testcase,
and qemu is starting to use this model when building device trees for some
targets such as e500.

However, the read/write functions require some sort of valid tree to begin
with, so this necessitates either having a trivial canned dtb to begin with
or, more commonly, creating an empty tree using the serial-write functions
first.

This patch adds a helper function which uses the serial-write functions to
create a trivial, empty but complete and valid tree in a supplied buffer,
ready for manipulation with the read/write functions.

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

tests/rw_tree1.c

@@ -57,17 +57,7 @@ int main(int argc, char *argv[])
 	fdt = xmalloc(SPACE);
 
 	/* First create empty tree with SW */
-	CHECK(fdt_create(fdt, SPACE));
-
-	CHECK(fdt_finish_reservemap(fdt));
-	CHECK(fdt_begin_node(fdt, ""));
-	CHECK(fdt_end_node(fdt));
-	CHECK(fdt_finish(fdt));
-
-	verbose_printf("Built empty tree, totalsize = %d\n",
-		       fdt_totalsize(fdt));
-
-	CHECK(fdt_open_into(fdt, fdt, SPACE));
+	CHECK(fdt_create_empty_tree(fdt, SPACE));
 
 	CHECK(fdt_add_mem_rsv(fdt, TEST_ADDR_1, TEST_SIZE_1));
 	CHECK(fdt_add_mem_rsv(fdt, TEST_ADDR_2, TEST_SIZE_2));