initcalls: Add initial implementation

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

include/drivers/serial.h

@@ -0,0 +1,27 @@
+/*
+    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.
+ */
+
+struct serial_dev {
+	//TODO add more functions and attributes here
+	int (*putc)(char);
+};
+
+int serial_register_device(struct serial_dev *sdev);
+struct serial_dev *serial_first_device();