update_chroot: only update llvm when binpkgs are available

BUG=chromium:642813
TEST=uprev llvm and run cros_sdk --delete and cros_sdk
     llvm was not updated.

Change-Id: Ic33a0ad96695516055ec0336ef6ed4f994934907
Reviewed-on: https://chromium-review.googlesource.com/380326
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed