vendor: lineage: Upgrade to LineageOS 15.1

 * Update backuptool versions
 * Update text references
 * Set version minor to 1

Change-Id: I30eea1f1cb01e2165311364cc62b589669bb1290
diff --git a/prebuilt/common/bin/backuptool.functions b/prebuilt/common/bin/backuptool.functions
index dd867c0..de6eb42 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=15.0
+export V=15.1
 
 copy_file() {
   cp -dp "$1" "$2"