fix fwup_public_keys including bad arg

lib/nerves_hub_link/configurator.ex

@@ -105,14 +105,22 @@ defmodule NervesHubLink.Configurator do
   end
 
   defp add_fwup_public_keys(config) do
-    fwup_public_keys = NervesHubLink.Certificate.fwup_public_keys()
+    # NervesHubLink.Certificate.fwup_public_keys() is compiled into the module
+    # This is a simple workaround to support changing hardcoded binary keys
+    # in the config and being able to load without recompiling. However, it
+    # is still suggested to recompile as well which is required for resolve
+    # public keys referenced by an atom
+    fwup_public_keys =
+      for key <- NervesHubLink.Certificate.fwup_public_keys() ++ config.fwup_public_keys,
+          is_binary(key),
+          do: key
 
     if fwup_public_keys == [] do
       Logger.error("No fwup public keys were configured for nerves_hub_link.")
       Logger.error("This means that firmware signatures are not being checked.")
       Logger.error("nerves_hub_link will fail to apply firmware updates.")
     end
 
-    %{config | fwup_public_keys: config.fwup_public_keys ++ fwup_public_keys}
+    %{config | fwup_public_keys: fwup_public_keys}
   end
 end

test/nerves_hub_link/configurator_test.exs

@@ -20,4 +20,21 @@ defmodule NervesHubLink.ConfiguratorTest do
     config = NervesHubLink.Configurator.build()
     assert Map.has_key?(config.params, "fwup_version")
   end
+
+  test "only includes binary in fwup_public_keys" do
+    keys = [
+      "thisisavalidkey==",
+      :not_valid,
+      false,
+      {:also, :not, :valid},
+      nil,
+      1234
+    ]
+
+    Application.put_env(:nerves_hub_link, :fwup_public_keys, keys)
+
+    config = NervesHubLink.Configurator.build()
+
+    assert ["thisisavalidkey=="] == config.fwup_public_keys
+  end
 end