presence: Add presence sensor interface

Prepare to extend the fan presence model to include
sensors of other types with a presence sensor interface
class.

Presence sensors can be both event driven or polled.  Provide
start and stop for the former and present for the latter.

Change-Id: Ia1b91a5e30e27fbe66c6f075e5e5ce620dac997e
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
1 file changed