Sercomm partition map support

[Nathan Lutchansky]

I got lucky (?) and purchased a Netgear WAC124 with bad erase blocks in 
the NAND flash in both the kernel and rootfs partitions.  I did not know 
this, of course, and attempted to flash OpenWrt 21.02.1, which immediately 
resulted in a bricked device.  Fortunately, nmrpflash could restore the 
vendor firmware, and after a little investigation I found the issue.

There is a patch[1] by Jan Hoffmann <jan at 3e8.eu> based on work by NOGUCHI 
Hiroshi <drvlabo at gmail.com> to parse the Sercomm partition map, which I 
have confirmed fixes the problem.  (The factory writes a modified 
partition map to devices with bad blocks present at manufacturing time.) 
The patch was first proposed in June, 2020[2] and again in February, 
2021[3].  May I suggest it be applied to both master and the 21.02 branch, 
where it applies cleanly?  Without it, there is a non-zero chance somebody 
will brick their router by trying to install OpenWrt, due to a known bug, 
on devices that are advertised to be supported.

I am happy to provide logs or testing if needed.  I am also happy to mail 
this thing to an OpenWrt developer (for free) if they would like it for 
their own testing.  I even added a serial port header...  -Nathan


[1] https://github.com/janh/openwrt/tree/sercomm-partition-table-v2
[2] http://lists.openwrt.org/pipermail/openwrt-devel/2020-June/029857.html
[3] http://lists.openwrt.org/pipermail/openwrt-devel/2021-February/034033.html