Driver model: improve support for system devices.

  
- Create struct sys_device to describe system-level devices (CPUs, PICs, etc.). This 
  structure includes a 'name' and 'id' field for drivers to fill in with a simple
  canonical name (like 'pic' or 'floppy') and the id of the device relative to its 
  discovery in the system (it's enumerated value).

  The core then constructs the bus_id for the device from these, giving them meaningful
  names when exporting them to userspace:

# tree -d /sys/root/sys/
/sys/root/sys/
|-- pic0
`-- rtc0

- Replace 
	int register_sys_device(struct device * dev);
	with 
	int sys_device_register(struct sys_device * sysdev);

- Fixup the users of the API.

- Add a system_bus_type for devices to associate themselves with. This provides a 
  bus/system/ directory in driverfs that looks like:

# tree -d /sys/bus/system/
/sys/bus/system/
|-- devices
|   |-- pic0 -> ../../../root/sys/pic0
|   `-- rtc0 -> ../../../root/sys/rtc0
`-- drivers
    `-- pic

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