[FS#1429] OpenWrt kernel is missing support for hardware rtc devices

[LEDE Bugs]

The following task has a new comment added:

FS#1429 - OpenWrt kernel is missing support for hardware rtc devices
User who did this - Robert Marko (robimarko)

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This PR should fix this and enable buildbots to also build RTC kmods.
https://github.com/openwrt/openwrt/pull/797
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More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=1429#comment4464