Some versions [1] of the Lenovo ThinkPad Compact USB Keyboard with TrackPoint USB
have the pointing stick on an event node that has keys but is not a regular
keyboard. Thus the stick falls through the cracks and gets disabled on tablet
mode switch. Instead of adding more hacks let's do this properly: tag the
pointing stick as external and have the code in place to deal with that.

[1] This may be caused by recent kernel changes

Fixes #291



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>