Support wiping userdata for A/B OTA packages.

update_engine now accepts POWERWASH=1 to schedule a factory reset in
the post-install phase. Hook up with the --wipe_user_data flag in the
OTA script.

Bug: 28700985
Change-Id: Ie73876a61db90d124d2af588d674757376e9aabc

tools/releasetools/ota_from_target_files.py

@@ -1276,6 +1276,11 @@ def WriteABOTAPackageWithBrilloScript(target_file, output_file,
   p1.wait()
   assert p1.returncode == 0, "brillo_update_payload properties failed"
 
+  if OPTIONS.wipe_user_data:
+    with open(properties_file, "a") as f:
+      f.write("POWERWASH=1\n")
+    metadata["ota-wipe"] = "yes"
+
   # Add the signed payload file and properties into the zip.
   common.ZipWrite(output_zip, properties_file, arcname="payload_properties.txt")
   common.ZipWrite(output_zip, signed_payload_file, arcname="payload.bin",