Found this, not sure it still makes sense:

// TODO move to migration kit
object OldConfigurationLoader {

val defaultConfig: Config = {
val cfg = fromProperties orElse fromClasspath orElse fromHome getOrElse emptyConfig
cfg.withFallback(ConfigFactory.defaultReference).resolve(ConfigResolveOptions.defaults)
}

// file extensions (.conf, .json, .properties), are handled by parseFileAnySyntax
val defaultLocation: String = (systemMode orElse envMode).map("akka." + _).getOrElse("akka")

private def envMode = System.getenv("AKKA_MODE") match {
case null | "" ⇒ None
case value ⇒ Some(value)
}

private def systemMode = System.getProperty("akka.mode") match {
case null | "" ⇒ None
case value ⇒ Some(value)
}

private def configParseOptions = ConfigParseOptions.defaults.setAllowMissing(false)

private def fromProperties = try {
val property = Option(System.getProperty("akka.config"))
property.map(p ⇒
ConfigFactory.systemProperties.withFallback(
ConfigFactory.parseFileAnySyntax(new File(p), configParseOptions)))
} catch { case _ ⇒ None }

private def fromClasspath = try {
Option(ConfigFactory.systemProperties.withFallback(
ConfigFactory.parseResourcesAnySyntax(ActorSystem.getClass, "/" + defaultLocation, configParseOptions)))
} catch { case _ ⇒ None }

private def fromHome = try {
Option(ConfigFactory.systemProperties.withFallback(
ConfigFactory.parseFileAnySyntax(new File(GlobalHome.get + "/config/" + defaultLocation), configParseOptions)))
} catch { case _ ⇒ None }

private def emptyConfig = ConfigFactory.systemProperties
}