[FS#378] Flash new firmware image success by u-boot only

[LEDE Bugs]

The following task has a new comment added:

FS#378 - Flash new firmware image success by u-boot only
User who did this - qingzi (qingzi)

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need wait for ttl to usb device, on express.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=378#comment1198