base-files: use jshn lib for ubus sysupgrade argument generation

With this change the well known jshn library will be used, to build the
json arguments for the ubus sysupgrade method. This is also used in all
other shell program that uses JSON. This commit unifies that.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

package/base-files/files/sbin/sysupgrade

@@ -360,18 +360,15 @@ if [ -n "$FAILSAFE" ]; then
 	printf '%s\x00%s\x00%s' "$RAM_ROOT" "$IMAGE" "$COMMAND" >/tmp/sysupgrade
 	lock -u /tmp/.failsafe
 else
-	force_attr=""
-	[ $FORCE -eq 1 ] && force_attr="\"force\": true,"
-	backup_attr=""
-	[ $SAVE_CONFIG -eq 1 ] && backup_attr="\"backup\": $(json_string $CONF_TAR),"
-	ubus call system sysupgrade "{
-		\"prefix\": $(json_string "$RAM_ROOT"),
-		\"path\": $(json_string "$IMAGE"),
-		$force_attr
-		$backup_attr
-		\"command\": $(json_string "$COMMAND"),
-		\"options\": {
-			\"save_partitions\": $SAVE_PARTITIONS
-		}
-	}"
+	json_init
+	json_add_string prefix "$RAM_ROOT"
+	json_add_string path "$IMAGE"
+	[ $FORCE -eq 1 ] && json_add_boolean force 1
+	[ $SAVE_CONFIG -eq 1 ] && json_add_string backup "$CONF_TAR"
+	json_add_string command "$COMMAND"
+	json_add_object options
+	json_add_int save_partitions "$SAVE_PARTITIONS"
+	json_close_object
+
+	ubus call system sysupgrade "$(json_dump)"
 fi