backuptool: only restore from same major.minor version

Change-Id: I63113fa9b3cbc6b0e1b3df75ef7dfb3160ee5062
diff --git a/prebuilt/common/bin/backuptool.functions b/prebuilt/common/bin/backuptool.functions
index 833460f..ef8479a 100644
--- a/prebuilt/common/bin/backuptool.functions
+++ b/prebuilt/common/bin/backuptool.functions
@@ -5,7 +5,7 @@
 
 export C=/tmp/backupdir
 export S=/system
-export V=9
+export V=10.1
 
 backup_file() {
   if [ -e "$1" ]; then