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

bpttool

diff --git a/bpttool b/bpttool
index 6b4f56d..31cc508 100755
--- a/bpttool
+++ b/bpttool
@@ -946,7 +946,7 @@
       BptError: If another application-specific error occurs
     """
 
-    partitions, _ = self._read_json([input_file], 'ab_collapse')
+    partitions, _ = self._read_json([input_file], ab_collapse)
 
     part = None
     for p in partitions: