update_payload: Make paycheck take "root" instead of "rootfs"

This commit makes paycheck take "root" as the rootfs partition label
instead of "rootfs", which makes it consistent with other tooling like
delta_generator.

BUG=chromium:794404
TEST=test_paycheck.sh and run_unittests

Change-Id: I20835e93adbcde459bc86d832e695b38bca55e38
Reviewed-on: https://chromium-review.googlesource.com/1132843
Commit-Ready: Tudor Brindus <tbrindus@chromium.org>
Tested-by: Tudor Brindus <tbrindus@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/scripts/update_payload/checker.py b/scripts/update_payload/checker.py
index d3230cc..746d4be 100644
--- a/scripts/update_payload/checker.py
+++ b/scripts/update_payload/checker.py
@@ -628,11 +628,11 @@
                           'signatures_offset', 'signatures_size', 'manifest')
 
     if self.major_version == 1:
-      for part in common.CROS_PARTITIONS:
-        self.old_part_info[part] = self._CheckOptionalSubMsg(
-            manifest, 'old_%s_info' % part, report)
-        self.new_part_info[part] = self._CheckMandatorySubMsg(
-            manifest, 'new_%s_info' % part, report, 'manifest')
+      for real_name, proto_name in common.CROS_PARTITIONS:
+        self.old_part_info[real_name] = self._CheckOptionalSubMsg(
+            manifest, 'old_%s_info' % proto_name, report)
+        self.new_part_info[real_name] = self._CheckMandatorySubMsg(
+            manifest, 'new_%s_info' % proto_name, report, 'manifest')
 
       # Check: old_kernel_info <==> old_rootfs_info.
       self._CheckPresentIff(self.old_part_info[common.KERNEL].msg,
@@ -647,7 +647,7 @@
             part, 'new_partition_info', report, 'manifest.partitions')
 
       # Check: Old-style partition infos should not be specified.
-      for part in common.CROS_PARTITIONS:
+      for _, part in common.CROS_PARTITIONS:
         self._CheckElemNotPresent(manifest, 'old_%s_info' % part, 'manifest')
         self._CheckElemNotPresent(manifest, 'new_%s_info' % part, 'manifest')