dev: Add support for i2c devices

This patch adds an I2C bus and base device. I2C is used to connect a
variety of sensors, and this patch serves as a starting point to
enable a range of I2C devices.

src/dev/SConscript

@@ -43,6 +43,7 @@ SimObject('BadDevice.py')
 SimObject('CopyEngine.py')
 SimObject('DiskImage.py')
 SimObject('Ethernet.py')
+SimObject('I2C.py')
 SimObject('Ide.py')
 SimObject('Pci.py')
 SimObject('Platform.py')
@@ -61,6 +62,7 @@ Source('etherint.cc')
 Source('etherlink.cc')
 Source('etherpkt.cc')
 Source('ethertap.cc')
+Source('i2cbus.cc')
 Source('i8254xGBe.cc')
 Source('ide_ctrl.cc')
 Source('ide_disk.cc')