ostree-prepare-root as init: exec init from deployment if run as PID1

This supports running ostree on embedded platforms without an initrd.
Specificially I'm trying to do bringup on an NVidia Tegra based Jetson TK1
dev board.

Closes: #403
Approved by: cgwalters

src/switchroot/ostree-prepare-root.c

@@ -156,12 +156,10 @@ main(int argc, char *argv[])
   int orig_cwd_dfd;
 
   if (argc < 2)
-    {
-      fprintf (stderr, "usage: ostree-prepare-root SYSROOT\n");
-      exit (EXIT_FAILURE);
-    }
+    root_mountpoint = "/";
+  else
+    root_mountpoint = argv[1];
 
-  root_mountpoint = argv[1];
   deploy_path = resolve_deploy_path (root_mountpoint);
 
   /* Create a temporary target for our mounts in the initramfs; this will
@@ -303,6 +301,15 @@ main(int argc, char *argv[])
       perrorv ("failed to MS_MOVE %s to %s", deploy_path, root_mountpoint);
       exit (EXIT_FAILURE);
     }
-  
-  exit (EXIT_SUCCESS);
+
+  if (getpid() == 1)
+    {
+      execl ("/sbin/init", "/sbin/init", NULL);
+      perrorv ("failed to exec init inside ostree");
+      exit (EXIT_FAILURE);
+    }
+  else
+    {
+      exit (EXIT_SUCCESS);
+    }
 }