I2C Device Simulation

[Kevin Townsend <ke...@adafruit.com.INVALID>]

I pushed out a simple PR to add I2C stubs for the simulator 
(https://github.com/apache/mynewt-core/pull/1251), but the end goal is 
to try to simulate specific sensors via callbacks as naturally as 
possible to transparently switch between sim and HW targets.

There needs to be some sort of callback system added to the sim 
hal_i2c.c code where you can register a specific I2C address, and 
redirect the read/write/probe calls up to the sensor driver package, 
where they can be implemented as drivername_sim.c or something like 
that. This should keep the code relatively clean, and the simulation 
code is held in an appropriate package.

Are there any specific suggestions on what would be the best way to 
handle this, though, either implementing the registration and callbacks 
or perhaps another solution I'm not seeing here? There currently isn't a 
good example in the codebase of simulating an actual sensor or external 
device, and I think it would make a good addition to the project to show 
how you can model a specific IC in the simulator.

Kevin