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/common/utils.cc b/common/utils.cc
index 91dcfc8..b4956e7 100644
--- a/common/utils.cc
+++ b/common/utils.cc
@@ -613,9 +613,14 @@
 
 bool MountFilesystem(const string& device,
                      const string& mountpoint,
-                     unsigned long mountflags) {  // NOLINT(runtime/int)
-  // TODO(sosa): Remove "ext3" once crbug.com/208022 is resolved.
-  const vector<const char*> fstypes{"ext2", "ext3", "ext4", "squashfs"};
+                     unsigned long mountflags,  // NOLINT(runtime/int)
+                     const string& type) {
+  vector<const char*> fstypes;
+  if (type.empty()) {
+    fstypes = {"ext2", "ext3", "ext4", "squashfs"};
+  } else {
+    fstypes = {type.c_str()};
+  }
   for (const char* fstype : fstypes) {
     int rc = mount(device.c_str(), mountpoint.c_str(), fstype, mountflags,
                    nullptr);
@@ -625,7 +630,9 @@
     PLOG(WARNING) << "Unable to mount destination device " << device
                   << " on " << mountpoint << " as " << fstype;
   }
-  LOG(ERROR) << "Unable to mount " << device << " with any supported type";
+  if (!type.empty()) {
+    LOG(ERROR) << "Unable to mount " << device << " with any supported type";
+  }
   return false;
 }