calculate rootfs usage instead of overlayfs

in case a target does not use overlayfs

package/gluon-announce/files/lib/gluon/announce/statistics.d/overlay_usage

@@ -1,5 +0,0 @@
-local fs = require "nixio.fs"
-local opkg = require "luci.model.ipkg"
-
-local st = fs.statvfs(opkg.overlay_root())
-return 1 - st.bfree / st.blocks

package/gluon-announce/files/lib/gluon/announce/statistics.d/rootfs_usage

@@ -0,0 +1,4 @@
+local fs = require "nixio.fs"
+
+local st = fs.statvfs("/")
+return 1 - st.bfree / st.blocks