enter_chroot: clean out stale ssh dirs

Over time, stale ssh agent dirs build up in /tmp.  Have enter_chroot run
a simple rmdir to clean out any empty dirs.  Since we mount over top the
dir, this shouldn't kill any valid mount points.

BUG=None
TEST=`cros_sdk` cleaned out empty ssh dirs in /tmp

Change-Id: Ib9f063f99db61825082818a39a39c5eb01f2d24e
Reviewed-on: https://gerrit.chromium.org/gerrit/39004
Reviewed-by: David James <davidjames@chromium.org>
Reviewed-by: Matt Tennant <mtennant@chromium.org>
Reviewed-by: Brian Harring <ferringb@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/sdk_lib/enter_chroot.sh b/sdk_lib/enter_chroot.sh
index f200335..23078ff 100755
--- a/sdk_lib/enter_chroot.sh
+++ b/sdk_lib/enter_chroot.sh
@@ -282,6 +282,9 @@
     fi
 
     if [ $FLAGS_ssh_agent -eq $FLAGS_TRUE ]; then
+      # Clean up previous ssh agents.
+      rmdir "${FLAGS_chroot}"/tmp/ssh-* 2>/dev/null
+
       if [ -n "${SSH_AUTH_SOCK}" -a -d "${SUDO_HOME}/.ssh" ]; then
         TARGET_DIR="${FLAGS_chroot}/home/${SUDO_USER}/.ssh"
         user_mkdir "${TARGET_DIR}"