[FS#1005] sysupgrade should stop network service

[LEDE Bugs]

The following task has a new comment added:

FS#1005 - sysupgrade should stop network service
User who did this - Baptiste Jonglez (bjonglez)

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Actually, responding to ping is already a useful indication: it means that the device is still sysupgrading.  When it stops pinging, it means that sysupgrade is done and the device is rebooting.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1005#comment3471