Make ROOT-B 4096 blocks long to workaround recovery image padding

BUG=chrome-os-partner:15036
TEST=Build/run recovery image, verify it runs at expected speed

Change-Id: I8bd701761456a9444845ffc87b9b70f7d3f0c371
Reviewed-on: https://gerrit.chromium.org/gerrit/35226
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Ready: Liam McLoughlin <lmcloughlin@chromium.org>
Tested-by: Liam McLoughlin <lmcloughlin@chromium.org>
(cherry picked from commit 61de2dc827e7ea82149f97a26d77827b888c266e)
Reviewed-on: https://gerrit.chromium.org/gerrit/35366
Tested-by: Katie Roberts-Hoffman <katierh@chromium.org>
Reviewed-by: Ryan Cairns <rtc@chromium.org>
diff --git a/build_library/legacy_disk_layout.json b/build_library/legacy_disk_layout.json
index 6ae68d6..71a6644 100644
--- a/build_library/legacy_disk_layout.json
+++ b/build_library/legacy_disk_layout.json
@@ -171,7 +171,7 @@
         "num": 5,
         "label":"ROOT-B",
         "type":"rootfs",
-        "blocks":"1"
+        "blocks":"4096"
       }
     ],
     "pgo": [