Kobo only mounts in /mnt/, set filter in case user uses some custom mount points (#4981)

Normally only mmcblk* stuff (internal/external) will get mounted to /mnt/onboard or /mnt/sdcard, but it can sometimes be useful, see: https://github.com/koreader/koreader/pull/4981#issuecomment-487670972

plugins/systemstat.koplugin/main.lua

@@ -16,8 +16,10 @@ local SystemStat = {
 }
 
 function SystemStat:init()
-    if Device:isCervantes() or Device:isKobo() or Device:isPocketBook() then
+    if Device:isCervantes() or Device:isPocketBook() then
         self.storage_filter = "mmcblk"
+    elseif Device:isKobo() then
+        self.storage_filter = " /mnt/"
     elseif Device:isKindle() then
         self.storage_filter = "' /mnt/us$'"
     elseif Device:isSDL() then