v1.15.1

This is a security/bug fix release that updates to Buildroot 2021.02.1 and OTP
23.3.1. It should be safe for everyone to apply.

• Improvements

  • espeak has been removed from the default install to trim 13 MB off the root
    filesystem

• Updated dependencies

  • nerves_system_br v1.15.1
  • Buildroot 2021.02
  • Erlang/OTP 23.3.1