AU: delta compress the kernel partition

Add kernel partition to proto buf. Change delta generator to compress
the kernel partition and the delta performer to perform operations to
update the kernel partition. Update unittests to test it.

Also, remove checksums from the protobuf while we're at it, since the
partitions themselves will contains checksums.

BUG=None
TEST=Attached unittest

Review URL: http://codereview.chromium.org/1819002
diff --git a/update_metadata.proto b/update_metadata.proto
index 115d513..2fa8392 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -90,10 +90,8 @@
     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];
 }