releasetools: Add prefix when dumping fingerprints.

We used to dump "Source: <fingerprint>" in update logs. The "Source: "
prefix was unintentionally dropped out.

Test: Check the generated incremental BBOTA script.
Change-Id: I4de62333aa38e3fb09a76df0e769b62af48e0313

tools/releasetools/ota_from_target_files.py

@@ -996,8 +996,8 @@ else if get_stage("%(bcb_dev)s") != "3/3" then
     script.Comment("Stage 1/3")
 
   # Dump fingerprints
-  script.Print(source_fp)
-  script.Print(target_fp)
+  script.Print("Source: %s" % (source_fp,))
+  script.Print("Target: %s" % (target_fp,))
 
   script.Print("Verifying current system...")