[FS#399] odhcpd: add self configuration for DHCP UCI option 'dns'

[LEDE Bugs]

The following task has a new comment added:

FS#399 - odhcpd: add self configuration for DHCP UCI option 'dns'
User who did this - Eric Luehrsen (EricLuehrsen)

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Please close to FS#635. It sufficiently meets the need. The options here are extra complexity that won't add much more value over the recent implementation.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=399#comment2284