v0.13.8

This release has many updates, but none of them are expected to be noticeable to
most Nerves users. Most are in support of the Raspberry Pi's TRYBOOT feature
which is not supported in official Nerves systems.

• Changes
  • Update licensing and copyright for REUSE compliance
  • Add Nerves.Runtime.FwopOps.status/1 to report the currently running slot
    and the one that will run on the next boot
  • Improve active (current) firmware slot detection when ops.fw is available.
    While nothing has changed in official Nerves systems, more documentation has
    been added here to make the ops.fw tasks use official.
  • Support reloading KV cache and reload it automatically on firmware
    operations that may affect it.
  • Return error and warning details from fwup calls to avoid losing failure
    reasons
  • Reduce Dialyzer warnings when calling Nerves.Runtime.mix_target/0 at
    runtime