SSH subsystem for upgrading Nerves devices

This module provides an SSH subsystem for Erlang's ssh application. This makes it possible to send firmware updates to Nerves devices using plain old ssh like this:

cat $firmware | ssh -s $ip_address fwup

Where $ip_address is the IP address of your Nerves device. Depending on how you have Erlang's ssh application set up, you may need to pass more parameters (like username, port, identities, etc.).

See nerves_ssh for an easy way to set this up. If you don't want to use nerves_ssh, then in your call to :ssh.daemon add the return value from SSHSubsystemFwup.subsystem_spec/1:

devpath = Nerves.Runtime.KV.get("nerves_fw_devpath")

:ssh.daemon([
      {:subsystems, [SSHSubsystemFwup.subsystem_spec(devpath: devpath)]}
    ])

See SSHSubsystemFwup.subsystem_spec/1 for options. You will almost always need to pass the path to the device that should be updated since that is device-specific.