base: make stack area base specifiable for core

When running core as the kernel inside every component, a separate
stack area for core is needed that is different from the user-land
component's one.

Ref #2091

repos/base/src/lib/base/stack_area_addr.cc

@@ -0,0 +1,17 @@
+/*
+ * \brief  Component-local stack area base address
+ * \author Stefan Kalkowski
+ * \date   2017-06-02
+ */
+
+/*
+ * 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.
+ */
+
+/* base-internal includes */
+#include <base/internal/stack_area.h>
+
+Genode::addr_t Genode::stack_area_virtual_base() { return 0x40000000UL; }