v1.9.1

This release pulls in security and bug fix updates from nerves_system_br.
Erlang/OTP is now at 22.1.1.

IMPORTANT: The IEx prompt was being output on the UART. This was inconsistent
with other Nerves/Raspberry Pi systems and has been fixed. If you want to
use the UART, you'll need to override the erlinit.config file.

• Bug fixes

  • Default to displaying the iex prompt on the HDMI port like the other
    Raspberry Pi systems
  • Compile in the I2C drivers so they're available early on for NervesKey and
    other use at initialization

• Updated dependencies

  • nerves_system_br v1.9.4
  • linux - update to the raspberrypi-kernel_1.20190925-1 tag