v0.13.0

This update replaces Nerves.Runtime.target/0 with
Nerves.Runtime.mix_target/0. It is not believed that the former function was
used much, but if you do use it, note the return value change in mix_target/0.

• Changes
  • Support customizable key-value store backends so that it's possible to store
    firmware and board metadata outside of U-Boot environment blocks. See the
    :kv_backend configuration option.
  • Support changing the firmware revert script path (see :revert_fw_path)
  • Decide host vs. target differences at compile time to avoid target-only
    or host-only logic and dependencies existing unnecessarily at run-time.