core: simplify initialization

This patch removes the 'Core_parent' and 'Core_pd_session', and reduces
the 'Core_env'.

repos/base/src/lib/base/main_thread_cap.cc

@@ -0,0 +1,24 @@
+/*
+ * \brief  Access to the component's initial thread capability
+ * \author Norman Feske
+ * \date   2017-05-10
+ */
+
+/*
+ * Copyright (C) 2017 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+/* Genode includes */
+#include <deprecated/env.h>
+
+/* base-internal includes */
+#include <base/internal/globals.h>
+#include <base/internal/parent_cap.h>
+
+Genode::Thread_capability Genode::main_thread_cap()
+{
+	return Genode::env_deprecated()->parent()->main_thread_cap();
+}