Busybox: Enable IPv6 ping & traceroute

...for some reason, these are not enabled by its config even though IPv6
is on by default.

Change-Id: Ie9763c35ca07326b400638eda65d737e72eb63fb
diff --git a/board/czechlight/common/busybox-config b/board/czechlight/common/busybox-config
new file mode 100644
index 0000000..dc6a9c5
--- /dev/null
+++ b/board/czechlight/common/busybox-config
@@ -0,0 +1,2 @@
+CONFIG_PING6=y
+CONFIG_TRACEROUTE6=y