base-files: move xor() from caldata extraction to functions.sh

The xor() function is defined in each of the caldata extraction
scripts for several targets. Move it to functions.sh to reduce
duplicate code.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

package/base-files/files/lib/functions.sh

@@ -17,6 +17,22 @@ NO_EXPORT=1
 LOAD_STATE=1
 LIST_SEP=" "
 
+# xor multiple hex values of the same length
+xor() {
+	local val
+	local ret="0x$1"
+	local retlen=${#1}
+
+	shift
+	while [ -n "$1" ]; do
+		val="0x$1"
+		ret=$((ret ^ val))
+		shift
+	done
+
+	printf "%0${retlen}x" "$ret"
+}
+
 append() {
 	local var="$1"
 	local value="$2"