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/update_engine.gyp b/update_engine.gyp
index 4254e9a..47727c8 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -486,6 +486,7 @@
             'payload_generator/graph_utils_unittest.cc',
             'payload_generator/inplace_generator_unittest.cc',
             'payload_generator/payload_file_unittest.cc',
+            'payload_generator/payload_generation_config_unittest.cc',
             'payload_generator/payload_signer_unittest.cc',
             'payload_generator/tarjan_unittest.cc',
             'payload_generator/topological_sort_unittest.cc',