releasetools: Only verify the blocks to be touched.

For incremental BBOTAs, we used to verify the integrity of all the
blocks in the source partition. In order to reduce the time cost under
recovery, this CL changes to only verify the blocks that will be touched
in the given OTA package (BBOTA >= 3 only). This is a trade-off between
performance and reliability.

Bug: 27813356
Change-Id: I3975ae6f461f0f7e58d24f1df7df46a449d2988b
diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index 4d0d8a5..861c485 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -996,9 +996,9 @@
 """ % bcb_dev)
 
   # Verify the existing partitions.
-  system_diff.WriteVerifyScript(script)
+  system_diff.WriteVerifyScript(script, touched_blocks_only=True)
   if vendor_diff:
-    vendor_diff.WriteVerifyScript(script)
+    vendor_diff.WriteVerifyScript(script, touched_blocks_only=True)
 
   script.Comment("---- start making changes here ----")