Rename br_image_addr to br_image_off.

The boot response returns a flash offset, not a flash address. This is
causing confusion and leading to crashes on some platforms which don't
have flash at address 0.

Rename the field to make it more clear what its purpose is; future
patches can start fixing up usages.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/sim/csupport/run.c b/sim/csupport/run.c
index 68cbbf4..5d6f651 100644
--- a/sim/csupport/run.c
+++ b/sim/csupport/run.c
@@ -52,7 +52,7 @@
 	flash_areas = adesc;
 	if (setjmp(boot_jmpbuf) == 0) {
 		res = boot_go(&rsp);
-		/* printf("boot_go result: %d (0x%08x)\n", res, rsp.br_image_addr); */
+		/* printf("boot_go off: %d (0x%08x)\n", res, rsp.br_image_off); */
 		return res;
 	} else {
 		return -0x13579;