enter_chroot: use /proc/mounts rather than `mount`

Since /etc/mtab could be stale, use /proc/mounts instead.

BUG=None
TEST=`cros_sdk` in diff terminals still works

Change-Id: I526e5173581820c6983fe3702493a0349c1232c3
Reviewed-on: https://gerrit.chromium.org/gerrit/22860
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
diff --git a/sdk_lib/enter_chroot.sh b/sdk_lib/enter_chroot.sh
index 5846dc1..b7f8621 100755
--- a/sdk_lib/enter_chroot.sh
+++ b/sdk_lib/enter_chroot.sh
@@ -109,8 +109,8 @@
 
   local mounted_path="${MOUNTED_PATH}$target"
 
-  case ${MOUNT_CACHE} in
-  *" on ${mounted_path} "*)
+  case " ${MOUNT_CACHE} " in
+  *" ${mounted_path} "*)
     # Already mounted!
     ;;
   *)
@@ -251,7 +251,7 @@
     fi
 
     debug "Mounting chroot environment."
-    MOUNT_CACHE=$(mount)
+    MOUNT_CACHE=$(echo $(awk '{print $2}' /proc/mounts))
     mount_queue_init
     queue_mount none "-t proc" /proc
     queue_mount none "-t sysfs" /sys