[SensorAPI] Default Sensor Config

[Kevin Townsend]

One issues I saw working on some sensor drivers (based on the simulated 
accel as a starting point) is that no 'default' config settings are 
currently being set, and it seems possible that a sensor might be 
initialised without calling the config function explicitly.

The config system is great (highly flexible and easy to understand), but 
would it be better to have a default config setting that is set when the 
sensor is initialized, or should running the config function at least 
once be considered mandatory and the assumption made that it is always 
being called? In that case, should we return an error if something like 
a read function is called but the sensor hasn't been configured yet to 
put it into a known state (and probably wake the sensor up since many 
devices will default to booting into power down mode)?