Add option to automatically unmount system before installing ZIP

This adds the option to automatically unmount the system partition
before installing a ZIP-file.
This option is enabled by default, This is usefull on SAR-devices
where system is mounted as system_root and ZIPs (i.e. Magisk)
try to unmount system instead of system_root and fail to install.

Change-Id: Id94ae658964679e23aa7fa137e07edf74ee073df
diff --git a/variables.h b/variables.h
index 76da715..24cde92 100644
--- a/variables.h
+++ b/variables.h
@@ -140,6 +140,7 @@
 #define TW_MILITARY_TIME            "tw_military_time"
 #define TW_USE_SHA2                 "tw_use_sha2"
 #define TW_NO_SHA2                  "tw_no_sha2"
+#define TW_UNMOUNT_SYSTEM           "tw_unmount_system"
 
 // Also used:
 //   tw_boot_is_mountable