Only compile c code when necessary

mix.exs

@@ -23,7 +23,7 @@ defmodule Farmbot.Mixfile do
       app: :farmbot,
       description: "The Brains of the Farmbot Project",
       package: package(),
-      compilers: [:elixir_make] ++ Mix.compilers,
+      compilers: compilers(),
       make_clean: ["clean"],
       test_coverage: [tool: ExCoveralls],
       version: @version,
@@ -75,6 +75,14 @@ defmodule Farmbot.Mixfile do
     ]
   end
 
+  defp compilers do
+    case :init.get_plain_arguments() |> List.last() do
+      a when a in ['mix', 'compile', 'firmware'] ->
+        [:elixir_make] ++ Mix.compilers
+      _ -> Mix.compilers
+    end
+  end
+
   defp deps do
     [
       {:nerves, "~> 0.8.3", runtime: false},