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/delta_performer.cc b/payload_consumer/delta_performer.cc
index f95679c..f490c08 100644
--- a/payload_consumer/delta_performer.cc
+++ b/payload_consumer/delta_performer.cc
@@ -790,6 +790,12 @@
     install_part.name = partition.partition_name();
     install_part.run_postinstall =
         partition.has_run_postinstall() && partition.run_postinstall();
+    if (install_part.run_postinstall) {
+      install_part.postinstall_path =
+          (partition.has_postinstall_path() ? partition.postinstall_path()
+                                            : kPostinstallDefaultScript);
+      install_part.filesystem_type = partition.filesystem_type();
+    }
 
     if (partition.has_old_partition_info()) {
       const PartitionInfo& info = partition.old_partition_info();