your haxchi have a config file, right?
do you know your config file content?
it should let you launch a homebrew instead of sysNAND CFW by pressing a button.
You could for example launch the homebrew launcher. (press "a" button ?)
from here you could use Dimok custom firmware patcher (it doesn't have a name yet) to reboot in sysNAND with additional patches, it will of course re-launch haxchi but this time once you load homebrew launcher you should be able to launch ftpii everywhere.
using a FTP client you browse to your coldboot file (the xml ?) and restore the original file you backup'd (you did, right? if not, someone should have a copy).
That's only one idea, I didn't test it, and I'm not sure it will work. just something I just thought based on the knowledge I have after reading existing threads.
someone else should confirm before you try it !
edit:
the issue is not haxchi (reinstalling haxchi will still reboot-loop, except if you don't load sysNAND by default but then exiting to home menu will just re-loop)
Or maybe there's a new coldboot method? maybe I missed some recent progress
edit2:
I see there's a new "cfw-coldboot" fw.img creation mode, so I'm outdated. Don't follow my advices, they weren't meant for that method. I'll have to learn more about it.
edit3:
it seems I was right, using ftpii_everywhere is the way to uninstall haxchi coldboot for the moment. but if you were using "cfw-coldboot" fw.img, you already have the proper patches and don't need to reboot to dimok's sig path firmware, you are already using a compatible one! just boot to homebrew launcher by pressing "a" and launch ftpii everywhere.
there's a backup of the original system.xml on NAND:
The backup is called syshax.xml and it is placed in /vol/system/config. So you just download syshax.xml to your computer, rename it to system.xml and re-upload it back to the WiiU by overwriting the system.xml file