[FS#3540] ip6806x serial console is mute once kernel takes control (r15355-19d7e73ecc)

[OpenWrt Bugs]

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A new Flyspray task has been opened.  Details are below. 

User who did this - Hannu Nyman (hnyman) 

Attached to Project - OpenWrt/LEDE Project
Summary - ip6806x serial console is mute once kernel takes control (r15355-19d7e73ecc)
Task Type - Bug Report
Category - Kernel
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - High
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - Background: I noticed with ipq806x R7800 build r15355-19d7e73ecc that I am unable to sysupgrade back to r15254-1302bee12a. (I had just upgraded from r15254-1302bee12a to r15355-19d7e73ecc, so sysupgrade used to work a few days ago.)

Alarming is that while trying to debug that sysupgrade, I noticed to my surprise that with r15355-19d7e73ecc the serial console access to R7800 works only during the u-boot phase, but once control is passed to Linux kernel, the serial console gets turned off. Output stops at:

```
   Entry Point:  42208000
   Verifying Checksum ... OK
   Loading Kernel Image ... OK
OK
mtdparts variable not set, see 'help mtdparts'
no partitions defined

defaults:
mtdids  : nand0=msm_nand
mtdparts: none
info: "mtdparts" not set
Using machid 0x136c from environment

Starting kernel ...
```

The firmware runs ok, SSH console works ok. But serial cable does not produce any output. 

Rebooting the device causes the normal u-boot message to be shown ok, but the output stops when kernel takes over.
That makes me to suspect that something has changed in the kernel parameters.

commit 98b86296e6 is pretty much the only one to touch the ipq806x config in the last few days.  
(other possibility is 77575d4c02 but that does not look likely.

This commit 98b86296e6 made this change:

CONFIG_CMDLINE_OVERRIDE=y


I suspect that it affects the normal booting of the other normal ipq806x devices by changing kernel cmdline and somehow disables serial access. 

I will debug further by turning that off, but thought to highlight my suspicions already now.

Ps. Debugging sysupgrade is now impossible as serial console is mute. :-(

Pps.
The option is also mentioned in a forum discussion starting here 
https://forum.openwrt.org/t/ipq806x-nss-build-netgear-r7800-tp-link-c2600-linksys-ea8500/82525/44


More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=3540

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