Background: issue http://crosbug.com/p/63171.

Currently most projects will try to create their initial factory board drop (chromeos-factory-board) by reference board, or previous generation of platform.

However this creates many legacy, or rarely used options that people are not aware of what it's planned for.

For example, 63171 occurred because kern_a_device and root_device for reef were changed from release image slots (mmcblk0p4 mmcblk0p5) to test image slots (mmcblk1p2 and mmcblk1p3).

Also, there are lots of properties like main_temp_sensor_index and temp_sensors_to_test that almost none of the boards really need them.

This creates an interesting question - should we put "default for test params" in pytests, or test list options, or individual test items?

Since we already have so many boards, my recommendation is to leave most options to pytest, and only keeping needed changes (i.e., those you can find difference between different project overlays) in test list.

We need a full clean up for this.