tools(tmux): support target devices

This introduces the following support:

    ./tools/tmux -t <user>@<host> out/linux_gcc_debug_arm64

It also does the following:

  * Runs the script through `shellcheck`
    * `shellcheck` is a linter for shell files and checks conformance
  * Formats the script with `shfmt`
    * Provides a consistent style for the script
    * `shfmt -i 2 -ci -w tools/tmux`
  * Allows arguments to the script
    * `-c`: configuration
    * `-t`: target
    * `-C`: the build directory (similar to `ninja`)
  * Allows build directory as trailing argument, similar to other tools
  * Pulls the trace into `trace.protobuf` for easy uploading to UI
  * Adds documentation about target `tmux` support

Change-Id: Ic422de55fe7c59cb10cf53f241522cfe46dfea31