v1.21.0

This release includes a major change to the display drivers by switching to the
full KMS drivers. Raspberry Pi OS has been using these and they appear to be a
big improvement over the fake KMS ones. This might cause some application
changes or updates if you're using the display.

• Changes

  • Switch from using the fake KMS to full KMS device drivers for using the
    display. See PR #155.
  • Support aarch64 Linux builds
  • Add libdtc to support runtime loading of device tree overlays

• Updated dependencies

  • nerves_system_br v1.21.1
    and also see nerves_system_br v1.21.0
  • Buildroot 2022.08.1
  • Erlang/OTP 25.1.1