[FS#1109] AR71xx / TP-Link TL-WA850RE v2: no image build in current trunk

[LEDE Bugs]

The following task has a new comment added:

FS#1109 - AR71xx / TP-Link TL-WA850RE v2: no image build in current trunk
User who did this - donvipre (donvipre)

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it went broken in commit: 8464d2f64a552793bc9dd8ccfa0603ee17f0d3e6
where the kernel for ar71xx got updated to 4.9
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1109#comment3681