Parse postinstall parameters from the payload metadata.

Payload v2 includes a description of the post-install command it should
run, while in payload v1 we use the default values. This patch mounts
the partition on the new top-level directory called /postinstall that
should already be created.

Bug: 27177071
TEST=FEATURES=test emerge-link update_engine

Change-Id: Iaedf3b01e5e1ad57c68bd316b4b6e79cbab35bb6
diff --git a/payload_consumer/install_plan.h b/payload_consumer/install_plan.h
index d2f15fa..454dd78 100644
--- a/payload_consumer/install_plan.h
+++ b/payload_consumer/install_plan.h
@@ -90,8 +90,11 @@
     uint64_t target_size{0};
     brillo::Blob target_hash;
 
-    // Whether we should run the postinstall script from this partition.
+    // Whether we should run the postinstall script from this partition and the
+    // postinstall parameters.
     bool run_postinstall{false};
+    std::string postinstall_path;
+    std::string filesystem_type;
   };
   std::vector<Partition> partitions;