common/image.c: align usage of fdt_high with initrd_high

The commit message of a28afca (Add uboot "fdt_high" enviroment variable)
states that fdt_high behaves similarly to the existing initrd_high.
But fdt_high actually has an outstanding difference from initrd_high.
The former specifies the start address, while the later specifies the
end address.

As fdt_high and initrd_high will likely be used together, it'd be nice
to have them behave same.  The patch changes the behavior of fdt_high
to have it aligned with initrd_high.

The document of fdt_high in README is updated with an example to
demonstrate the usage of this environment variable.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/common/image.c b/common/image.c
index 202c8a1..fbdc40a 100644
--- a/common/image.c
+++ b/common/image.c
@@ -1289,16 +1289,14 @@
 
 		if (((ulong) desired_addr) == ~0UL) {
 			/* All ones means use fdt in place */
-			desired_addr = fdt_blob;
+			of_start = fdt_blob;
+			lmb_reserve(lmb, (ulong)of_start, of_len);
 			disable_relocation = 1;
-		}
-		if (desired_addr) {
+		} else if (desired_addr) {
 			of_start =
 			    (void *)(ulong) lmb_alloc_base(lmb, of_len, 0x1000,
-							   ((ulong)
-							    desired_addr)
-							   + of_len);
-			if (desired_addr && of_start != desired_addr) {
+							   (ulong)desired_addr);
+			if (of_start == 0) {
 				puts("Failed using fdt_high value for Device Tree");
 				goto error;
 			}