AU: delta compress the kernel partition

src/platform/update_engine/update_metadata.proto

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Update file format: A delta update file contains all the deltas needed
 // to update a system from one specific version to another specific
 // version. The update format is represented by this struct pseudocode:
 // struct delta_update_file {
 //   char magic[4] = "CrAU";
 //   uint32 file_format_version = 1;
@@ skipping 72 matching lines @@
     // of bytes to read from the blocks we pass it.
     optional uint64 src_length = 5;
 
     repeated Extent dst_extents = 6;
     // byte length of dst, not necessarily block aligned. It's only used for
     // BSDIFF, because we need to fill in the rest of the last block
     // that bsdiff writes with '\0' bytes.
     optional uint64 dst_length = 7;
   }
   repeated InstallOperation install_operations = 1;
-  // The checksums of the install device before and after the install process.
-  optional string src_checksum = 2;
-  optional string dst_checksum = 3;
+  repeated InstallOperation kernel_install_operations = 2;
 
   // (At time of writing) usually 4096
-  optional uint32 block_size = 5 [default = 4096];
+  optional uint32 block_size = 3 [default = 4096];
 }