Undo change of variable name to string

I can't be sure how this happened, but a previous commit turned a
variable usage into a string literal by putting quotes around it.
There's no context clues as to why this happened, and it broke things.

Change-Id: Ibd936e96bc204622db5a07ce129d9f42c48293e6
Test: RaspberryPi 3 image building no longer backtraces.
Bug: None