Fix detection of engine in rake db:load_config

Broken by d1d7c86d

activerecord/lib/active_record/railtie.rb

@@ -44,7 +44,7 @@ class Railtie < Rails::Railtie # :nodoc:
       ActiveRecord::Tasks::DatabaseTasks.migrations_paths = Rails.application.paths['db/migrate'].to_a
       ActiveRecord::Tasks::DatabaseTasks.fixtures_path = File.join Rails.root, 'test', 'fixtures'
 
-      if defined?(ENGINE_PATH) && engine = Rails::Engine.find(ENGINE_PATH)
+      if engine = Rails::Engine.find(find_engine_path(APP_RAKEFILE))
         if engine.paths['db/migrate'].existent
           ActiveRecord::Tasks::DatabaseTasks.migrations_paths += engine.paths['db/migrate'].to_a
         end