Expose whether an IEx session is active or not

This is useful for avoiding operations that affect remote debug of a
device when someone is actively debugging. For example, this makes it
possible to avoid upgrading the NervesHub connection to what looks like
a better network interface, but actually isn't.

lib/nerves_hub_link.ex

@@ -20,6 +20,12 @@ defmodule NervesHubLink do
     Socket.check_connection(:socket)
   end
 
+  @doc """
+  Return whether there's currently an active console session
+  """
+  @spec console_active?() :: boolean()
+  defdelegate console_active?, to: Socket
+
   @doc """
   Current status of the update manager
   """

lib/nerves_hub_link/socket.ex

@@ -1,4 +1,6 @@
 defmodule NervesHubLink.Socket do
+  @moduledoc false
+
   use Slipstream
   require Logger
 
@@ -44,6 +46,14 @@ defmodule NervesHubLink.Socket do
     GenServer.call(__MODULE__, {:check_connection, type})
   end
 
+  @doc """
+  Return whether an IEx or other console session is active
+  """
+  @spec console_active?() :: boolean()
+  def console_active?() do
+    GenServer.call(__MODULE__, :console_active?)
+  end
+
   @impl Slipstream
   def init(config) do
     rejoin_after = Application.get_env(:nerves_hub_link, :rejoin_after, 5_000)
@@ -110,6 +120,10 @@ defmodule NervesHubLink.Socket do
     {:reply, connected?(socket), socket}
   end
 
+  def handle_call(:console_active?, _from, socket) do
+    {:reply, socket.assigns.iex_pid != nil, socket}
+  end
+
   @impl Slipstream
   def handle_cast(:reconnect, socket) do
     # See handle_disconnect/2 for the reconnect call once the connection is