delta_generator: Include postinstall calls in the payload major version 2.

Added a new flag --new_postinstall_config_file which takes a path to a key
value store config file and saves this information to the PartitionUpdate
field.

The config file looks like this:
RUN_POSTINSTALL_root=true
POSTINSTALL_PATH_root=postinstall
FILESYSTEM_TYPE_root=ext4

"root" can be changed to any partition name.

Bug: 24537566
TEST=Generated a payload v2 with postinstall.
TEST=Added unittest.

Change-Id: Ied3c7bc2cfb54f4bcc69207f1e5bd473f72024fe
diff --git a/payload_generator/payload_file.h b/payload_generator/payload_file.h
index 51584d6..7cc792a 100644
--- a/payload_generator/payload_file.h
+++ b/payload_generator/payload_file.h
@@ -93,6 +93,8 @@
 
     PartitionInfo old_info;
     PartitionInfo new_info;
+
+    PostInstallConfig postinstall;
   };
 
   std::vector<Partition> part_vec_;