[FS#758] factory image for ubnt er-x missing

[LEDE Bugs]

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FS#758 - factory image for ubnt er-x missing
User who did this: Daniel Golle (dangowrt)
Percent Complete: 50% -> 80%

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=758