Use rbind for ChromeOS source directory

I use a mount at src/build/images to stop image builds from repeatedly
filling up my SSD. The chroot needs to respect this.

TEST=cros_sdk
BUG=none

Change-Id: I5c7a26c3b4f263bd683d3a897e6edccb83187bda
Reviewed-on: https://gerrit.chromium.org/gerrit/47178
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Tested-by: Michael Spang <spang@chromium.org>
diff --git a/sdk_lib/enter_chroot.sh b/sdk_lib/enter_chroot.sh
index bbe9ba8..6aeb747 100755
--- a/sdk_lib/enter_chroot.sh
+++ b/sdk_lib/enter_chroot.sh
@@ -219,7 +219,7 @@
       fi
     fi
 
-    setup_mount "${FLAGS_trunk}" "--bind" "${CHROOT_TRUNK_DIR}"
+    setup_mount "${FLAGS_trunk}" "--rbind" "${CHROOT_TRUNK_DIR}"
 
     debug "Setting up referenced repositories if required."
     REFERENCE_DIR=$(git config --file  \