Recovering all data from Xperia Z2 stuck in reboot loop

Hi there.

I updated my phone the other day and it ended up doing reboots with an error message regarding the update process. Before I hand it in for service, I wanted to make a copy of all the contents of the device..

So far, it looks like using some recovery kernel image and ramdisk is the right way to go, using "fastboot boot". What's a good image to use?

It's important that nothing is changed on the phone so I for example void the warranty.

Thanks,

Morten