Comment 37 for bug 1652270

I can also confirm that editing config.txt worked, but I had to remove the ppa (ppa:ubuntu-raspi2/ppa-rpi3) first.

If it can help somebody else, here is what I did.
I was stuck with the "starting kernel" message, just after boot.
1/ I removed the sd card from the raspberry and put it in another computer :

cd system-boot
mv initrd.img initrd.img.ori
mv initrd.img.bak initrd.img
mv bcm2710-rpi-3-b.dtb bcm2710-rpi-3-b.dtb.ori
mv bcm2710-rpi-3-b.dtb.bak bcm2710-rpi-3-b.dtb
mv boot.scr boot.scr.ori
mv boot.scr.bak boot.scr
mv vmlinuz vmlinuz.ori
mv vmlinuz.bak vmlinuz

2/ Put the sd card back in the raspberry, it should start up and uname -r should display 1034
3/ Comment the ppa:ubuntu-raspi2/ppa-rpi3 in /etc/apt/sources.list
sudo apt-get update
4/ Re-install kernel (from the offical Canonical rpi2)
sudo apt-get install --reinstall flash-kernel linux-firmware-raspi2
sudo reboot
5/ Normally you're stuck again at the boot but for another reason. So remove again the sd card from the raspberry and put it in another computer. This time modify the config.txt file in system-boot as follows :

# set extended DT area
#device_tree_address=0x100
#device_tree_end=0x8000
device_tree_address=0x02008000

6/ Now it should work. This also would mean that the special ppa for raspberry pi 3 is not anymore necessary. Currently my rpi3 work with kernel 1042. (+ Lubuntu 16.04)