close update package before installing; allow remount

Close the update package before invoking the binary, to allow the
installer to unmount /cache if it wants to.  Add a function to allow
remounting of a mount as read-only.

Change-Id: Idfcc96c3da66083295177f729263560be58034e4
diff --git a/mtdutils/mounts.h b/mtdutils/mounts.h
index 2e2765a..30b2927 100644
--- a/mtdutils/mounts.h
+++ b/mtdutils/mounts.h
@@ -28,4 +28,6 @@
 
 int unmount_mounted_volume(const MountedVolume *volume);
 
+int remount_read_only(const MountedVolume* volume);
+
 #endif  // MTDUTILS_MOUNTS_H_