Supplement base/log.h with raw output function

This patch introduces the Genode::raw function that prints output
directly via a low-level kernel mechanism, if available.

On base-linux, it replaces the former 'raw_write_str' function.
On base-hw, it replaces the former kernel/log.h interface.

Fixes #2012

repos/base-nova/src/include/base/internal/raw_write_string.h

@@ -0,0 +1,19 @@
+/*
+ * \brief  Kernel-specific raw-output back end
+ * \author Norman Feske
+ * \date   2016-03-08
+ */
+
+/*
+ * Copyright (C) 2016 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU General Public License version 2.
+ */
+
+#ifndef _INCLUDE__BASE__INTERNAL__RAW_WRITE_STRING_H_
+#define _INCLUDE__BASE__INTERNAL__RAW_WRITE_STRING_H_
+
+namespace Genode { void raw_write_string(char const *) { } }
+
+#endif /* _INCLUDE__BASE__INTERNAL__RAW_WRITE_STRING_H_ */