Implement sensor SM as an MMIO module about sancus-examples

[jovanbulck]

To simplify the example program, and to adhere to the description in the paper, the MMIO sensor module wouldn't use caller authentication. I.e., client modules would verify the integrity of the MMIO sensor SM, but not the other way around.

Such one-way callee sensor authentication could be useful in scenarios where exclusive access to a device is not desired, but rather a guarantee for remote stakeholders that the sensor data (e.g., a time stamp) was indeed authentic.