Make "Unmap Super Devices" option available to A-only devices

Change-Id: Ib3aed724b2e98f66387bdca240b0b4f719054594
(cherry picked from commit 2136fc5b91963fc21e782614ce6cfd55ca065eb9)
diff --git a/variables.h b/variables.h
index 8ae3469..cc6977c 100755
--- a/variables.h
+++ b/variables.h
@@ -147,6 +147,7 @@
 #define TW_NO_SHA2                  "tw_no_sha2"
 #define TW_UNMOUNT_SYSTEM           "tw_unmount_system"
 #define TW_IS_SUPER                 "tw_is_super"
+#define TW_VIRTUAL_AB_ENABLED       "tw_virtual_ab.enabled"
 #define TW_AUTO_REFLASHTWRP_VAR     "tw_auto_reflashtwrp"
 
 // Theme versioning