Create ota dir (#6857)

Partially reverts https://github.com/koreader/koreader/pull/6849

Needed for new installs, mainly in https://github.com/koreader/koreader/blob/master/frontend/ui/otamanager.lua#L173.
Fix https://github.com/koreader/koreader/pull/6849#issuecomment-723491945

datastorage.lua

@@ -54,7 +54,7 @@ local function initDataDir()
     local sub_data_dirs = {
         "cache", "clipboard",
         "data", "data/dict", "data/tessdata",
-        "history", "plugins",
+        "history", "ota", "plugins",
         "screenshots", "settings", "styletweaks",
     }
     for _, dir in ipairs(sub_data_dirs) do