Allow user to manually override enable_vabc_xor

--enable_vabc_xor=false to disable, --enable_vabc_xor=true or
--enable_vabc_xor to enable

Also the flag is now enabled by default. But update_engine will
still ignore XOR ops, as update_engine support for XOR is WIP.

Test: th
Change-Id: I038b0dda5cc9efcc1620e72c858626b56ffb1611
diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index 38af0d4..40da34d 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -221,6 +221,10 @@
       For VABC downgrades, we must finish merging before doing data wipe, and
       since data wipe is required for downgrading OTA, this might cause long
       wait time in recovery.
+
+  --enable_vabc_xor
+      Enable the VABC xor feature. Will reduce space requirements for OTA
+
 """
 
 from __future__ import print_function
@@ -286,7 +290,7 @@
 OPTIONS.disable_vabc = False
 OPTIONS.spl_downgrade = False
 OPTIONS.vabc_downgrade = False
-OPTIONS.enable_vabc_xor = False
+OPTIONS.enable_vabc_xor = True
 
 POSTINSTALL_CONFIG = 'META/postinstall_config.txt'
 DYNAMIC_PARTITION_INFO = 'META/dynamic_partitions_info.txt'
@@ -1312,7 +1316,7 @@
     elif o == "--vabc_downgrade":
       OPTIONS.vabc_downgrade = True
     elif o == "--enable_vabc_xor":
-      OPTIONS.enable_vabc_xor = True
+      OPTIONS.enable_vabc_xor = a.lower() != "false"
     else:
       return False
     return True
@@ -1357,7 +1361,7 @@
                                  "disable_vabc",
                                  "spl_downgrade",
                                  "vabc_downgrade",
-                                 "enable_vabc_xor",
+                                 "enable_vabc_xor=",
                              ], extra_option_handler=option_handler)
 
   if len(args) != 2: