recovery: Allow "Mount /system" for system_root_image.

When system images contain the root directory, there is no entry of
"/system" in the fstab. Change it to look for "/" instead if is true. We actually mount the partition
to /system_root instead, and create a symlink to /system_root/system
for /system. This allows "adb shell" to work properly.

Bug: 22855115
Change-Id: Ibac493a5a9320c98ee3b60bd2cc635b925f5454a
diff --git a/roots.h b/roots.h
index 230d9de..6e3b243 100644
--- a/roots.h
+++ b/roots.h
@@ -19,10 +19,6 @@
 #include "common.h"
-#ifdef __cplusplus
-extern "C" {
 // Load and parse volume data from /etc/recovery.fstab.
 void load_volume_table();
@@ -33,7 +29,10 @@
 // success (volume is mounted).
 int ensure_path_mounted(const char* path);
-// Make sure that the volume 'path' is on is mounted.  Returns 0 on
+// Similar to ensure_path_mounted, but allows one to specify the mount_point.
+int ensure_path_mounted_at(const char* path, const char* mount_point);
+// Make sure that the volume 'path' is on is unmounted.  Returns 0 on
 // success (volume is unmounted);
 int ensure_path_unmounted(const char* path);
@@ -46,8 +45,4 @@
 // mounted (/tmp and /cache) are mounted.  Returns 0 on success.
 int setup_install_mounts();
-#ifdef __cplusplus
 #endif  // RECOVERY_ROOTS_H_