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/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index c4a292a..74c297c 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -1288,7 +1288,7 @@
 
     /* Always boot from the primary slot. */
     rsp->br_flash_id = boot_img_fa_device_id(&boot_data, 0);
-    rsp->br_image_addr = boot_img_slot_off(&boot_data, 0);
+    rsp->br_image_off = boot_img_slot_off(&boot_data, 0);
     rsp->br_hdr = boot_img_hdr(&boot_data, slot);
 
  out: