Include Instructions in partition_tables /Readme.md to fix uboot before taking backup

[mabuch]

Hi

thank you very much for your work. I bought the Pinenote a long time ago and have not yet installed anything custom on it. Therefore I still had it as shipped from the facory. When I followed your instructions in https://github.com/PNDeb/pinenote-debian-image/blob/main/partition_tables/Readme.md I encountered a problem: my PineNote did not seem to boot after I followed the install instructions (screen stayed blank when turning it on).
After some research and a few tries I got it to work. My problem was, that I still had the faulty factory uboot and all backups taken (of sectors >32MB) were empty. Therefore I copied empty data back to the partitions (after changing the layout) and the PineNote would not do anything. Thanks to https://github.com/DorianRudolph/pinenotes and his backup I got it working in the end.

Please include instructions on how to fix uboot BEFORE the backup is taken. I guess it is as simple as to write the fixed uboot once to the device and then maybe reboot once again?
After reading through the README again I can see that it includes some information about the uboot 32Mb bug, but they are missing in the section of the flashing commands. But I did not understand what this section meant, I thought that the faulty uboot would be unable to boot linux or something like this. It was unclear to me, that I could not take a backup with this faulty version of uboot. I think it would be highly beneficial, if the steps to fix uboot were also included at the beginning of the flashing commands.

I also had too look elsewhere (https://pine64.org/documentation/PineNote/Development/Flashing/) on how to get to the stage where rkdeveloptool could be used (rebooting with the magnet placed correctly). It would also be nice, it this were a bit clearer from the instructions (a simple link mightd suffice), to make it easier for other beginners trying to install it.

Thank you very much for your work.