Calculate verity hash tree.

Calculate verity hash tree in FilesystemVerifierAction based on configs
specified in protobuf, and write it to target partition before reading
from those blocks.

A new error code kVerityCalculationError was added to report error if
it fails.

Bug: 28171891
Test: update_engine_unittests

Change-Id: I492885a0655bf51043902f578720fffd87e6a3cf
diff --git a/update_manager/chromeos_policy.cc b/update_manager/chromeos_policy.cc
index 71fec40..587ac67 100644
--- a/update_manager/chromeos_policy.cc
+++ b/update_manager/chromeos_policy.cc
@@ -87,6 +87,7 @@
     case ErrorCode::kUnsupportedMajorPayloadVersion:
     case ErrorCode::kUnsupportedMinorPayloadVersion:
     case ErrorCode::kPayloadTimestampError:
+    case ErrorCode::kVerityCalculationError:
       LOG(INFO) << "Advancing download URL due to error "
                 << chromeos_update_engine::utils::ErrorCodeToString(err_code)
                 << " (" << static_cast<int>(err_code) << ")";