initcalls: Add initial implementation

Create simple serial subsystem which makes use of initcalls, and convert
existing serial drivers to its use.

drivers/serial.c

@@ -0,0 +1,40 @@
+/*
+    Copyright (C) 2012, Aaron Lindsay <aaron@aclindsay.com>
+
+    This file is part of Aedrix.
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <init.h>
+#include <drivers/serial.h>
+
+struct serial_dev *first_serial_dev;
+
+int serial_register_device(struct serial_dev *sdev) {
+	if (!first_serial_dev)
+		first_serial_dev = sdev;
+	return 0;
+}
+
+struct serial_dev *serial_first_device() {
+	return first_serial_dev;
+}
+
+void serial_init() {
+	first_serial_dev = 0;
+}
+
+driversubsys_initcall(serial_init);