releasetools: Fallback to calculated fp if partition fp is missing

This happens when the fingerprint property is blacklisted for the
purposes of unified device builds.

Change-Id: I9f8a5041248c3f73fce7a16da73cb8f68c06c8e2

tools/releasetools/common.py

@@ -509,7 +509,7 @@ class BuildInfo(object):
     return self.GetBuildProp(key)
 
   def GetPartitionFingerprint(self, partition):
-    return self._partition_fingerprints.get(partition, None)
+    return self._partition_fingerprints.get(partition, self.CalculateFingerprint())
 
   def CalculatePartitionFingerprint(self, partition):
     try: