Please allow using a system variable for the garage doorstate
**Is your feature request related to a problem? Please describe.**
My Garage does use a different way to sense the doorstate. I was able to put the result in a system variable in my CCU. 

**Describe the solution you'd like**
I would llike to see the garage accessory being able to not only request states of CCU devices but also CCU system variables to detect the doorstate.

**Describe alternatives you've considered**
I tried every method I could think of and was not able to achieve a working setup with the current possibilities.