Fixes the following warning:
warning: boot-location: "new" is deprecated, use boot-location: modules

From the rpm-ostree docs (https://coreos.github.io/rpm-ostree/treefile/):

• “new”: A misnomer, this value is no longer “new”. Kernel data goes
  in /usr/lib/ostree-boot in addition to /usr/lib/modules. This is the
  default; use it if you have a need to care about upgrading from very
  old versions of libostree.

• “modules”: Kernel data goes just in /usr/lib/modules. Use this for
  new systems, and systems that don’t need to be upgraded from very
  old libostree versions.